Method for manufacturing storage battery electrode, storage battery electrode, storage battery, and electronic device

ABSTRACT

To provide a method for forming a storage battery electrode including an active material layer with high density in which the proportion of conductive additive is low and the proportion of the active material is high. To provide a storage battery having a higher capacity per unit volume of an electrode with the use of a storage battery electrode formed by the formation method. A method for forming a storage battery electrode includes the steps of forming a mixture including an active material, graphene oxide, and a binder; providing a mixture over a current collector; and immersing the mixture provided over the current collector in a polar solvent containing a reducer, so that the graphene oxide is reduced.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for manufacturing a storagebattery electrode.

2. Description of the Related Art

With the recent rapid spread of portable electronic devices such asmobile phones, smartphones, electronic books, and portable gamemachines, secondary batteries for drive power supply have beenincreasingly required to be smaller and to have higher capacity. Storagebatteries typified by lithium secondary batteries, which have advantagessuch as high energy density and high capacity, have been widely used assecondary batteries used for portable electronic devices.

A lithium secondary battery, which is one of storage batteries andwidely used due to its high energy density, includes a positiveelectrode including an active material such as lithium cobalt oxide(LiCoO₂) or lithium iron phosphate (LiFePO₄), a negative electrodeformed of a carbon material such as graphite capable of reception andrelease of lithium ions, a nonaqueous electrolyte in which anelectrolyte formed of a lithium salt such as LiBF₄ or LiPF₆ is dissolvedin an organic solvent such as ethylene carbonate or diethyl carbonate,and the like. A lithium secondary battery is charged and discharged insuch a way that lithium ions in the secondary battery are transferredbetween the positive electrode and the negative electrode through thenonaqueous electrolyte and intercalated into or deintercalated from theactive materials of the positive electrode and the negative electrode.

A binder is mixed into the positive electrode or the negative electrodein order that active materials can be bound or an active material and acurrent collector can be bound. Since the binder is generally an organichigh molecular compound such as polyvinylidene fluoride (PVDF) which hasan insulating property, the electric conductivity of the binder isextremely low. Therefore, as the ratio of the mixed binder to the activematerial is increased, the amount of the active material in theelectrode is relatively decreased, resulting in the lower dischargecapacity of the secondary battery.

Hence, by mixture of a conductive additive such as acetylene black (AB)or graphite particles, the electric conductivity between activematerials or between an active material and a current collector can beimproved. Thus, a positive electrode active material with high electricconductivity can be provided (see Patent Document 1).

REFERENCE

-   [Patent Document 1] Japanese Published Patent Application No.    2002-110162

SUMMARY OF THE INVENTION

However, because acetylene black used as a conductive additive is ahigh-volume particle with an average diameter of several tens ofnanometers to several hundreds of nanometers, contact between acetyleneblack and an active material hardly becomes surface contact and tends tobe point contact. Consequently, contact resistance between the activematerial and the conductive additive is high. Further, if the amount ofthe conductive additive is increased to increase contact points betweenthe active material and the conductive additive, the proportion of theamount of the active material in the electrode decreases, resulting inthe lower discharge capacity of the battery.

In the case where graphite particles are used as a conductive additive,natural graphite is generally used in consideration of cost. In thiscase, iron, lead, copper, or the like contained as an impurity in agraphite particle reacts with the active material or the currentcollector, which might reduce the potential or capacity of the battery.

Further, as particles of the active material become minuter, cohesionbetween the particles becomes stronger, which makes uniform dispersionin the binder or the conductive additive difficult. Consequently, aportion where active material particles are aggregated and denselypresent and a portion where active material particles are not aggregatedand thinly present are locally generated. In the portion where activematerial particles are aggregated and to which the conductive additiveis not mixed, the active material particles do not contribute toformation of the discharge capacity of the battery.

In view of the above problems, an object of one embodiment of thepresent invention is to provide a method for forming a storage batteryelectrode including an active material layer with high density in whichthe proportion of conductive additive is low and the proportion of theactive material is high. Another object of one embodiment of the presentinvention is to provide a storage battery having a higher capacity perunit volume of an electrode with the use of a storage battery electrodeformed by the formation method.

In view of the above objects, graphene is used instead of acetyleneblack or the like as a conductive additive included in an electrode ofone embodiment of the present invention. In particular, graphene oxidewith high dispersibility is used as a raw material and is mixed with anactive material and the like to form a mixture, the mixture is providedover a current collector, and then reduction treatment is performed, sothat an electrode including graphene as a conductive additive is formed.

Graphene is a carbon material having a crystal structure in whichhexagonal skeletons of carbon are spread in a planar form and is oneatomic plane extracted from graphite crystals. Due to its electrical,mechanical, or chemical characteristics which are surprisinglyexcellent, the graphene has been expected to be used for a variety offields of, for example, field-effect transistors with high mobility,highly sensitive sensors, highly-efficient solar cells, andnext-generation transparent conductive films and has attracted a greatdeal of attention.

Note that graphene in this specification refers to single-layer grapheneor multilayer graphene including two or more and hundred or less layers.Single-layer graphene refers to a one-atom-thick sheet of carbonmolecules having π bonds. Graphene oxide refers to a compound formed byoxidation of such graphene. When graphene oxide is reduced to formgraphene, oxygen contained in the graphene oxide is not entirelyreleased and part of the oxygen remains in the graphene. When thegraphene contains oxygen, the proportion of the oxygen is higher than orequal to 2 at. % and lower than or equal to 20 at. %, preferably higherthan or equal to 3 at. % and lower than or equal to 15 at. %.

In the case where graphene is multilayer graphene including grapheneobtained by reducing graphene oxide, the interlayer distance betweengraphenes is greater than or equal to 0.34 nm and less than or equal to0.5 nm, preferably greater than or equal to 0.38 nm and less than orequal to 0.42 nm, more preferably greater than or equal to 0.39 nm andless than or equal to 0.41 nm. In general graphite, the interlayerdistance between single-layer graphenes is 0.34 nm. Since the interlayerdistance between the graphenes used for the power storage device of oneembodiment of the present invention is longer than that in generalgraphite, carrier ions can easily transfer between the graphenes inmultilayer graphene.

In a storage battery electrode of one embodiment of the presentinvention, such graphenes are used as a conductive additive of theelectrode. However, in the case where a storage battery is formed insuch a manner that graphenes or graphenes formed by reducing grapheneoxides in advance (RGO (abbreviation of reduced graphene oxide)) aremixed with an active material and a binder, the graphenes or the RGOsaggregate in the electrode because of its low dispersibility and thus itis difficult to achieve favorable battery characteristics.

On the other hand, in the case of using graphene oxide as a raw materialof a conductive additive of an electrode, after a mixture formed bymixing graphene oxide, an active material, and a binder in a polarsolvent is provided over a current collector, the graphene oxide isreduced by reduction treatment, so that graphene can be formed. When anelectrode is formed using this method, a graphene network for electricconduction is formed in an active material layer including an activematerial and a binder. Thus, a storage battery electrode including ahighly conductive active material layer where active materials areelectrically connected to each other by graphene can be formed.

This is because graphene oxide used as a raw material of graphene is apolar material having a functional group such as an epoxy group, acarbonyl group, a carboxyl group, or a hydroxyl group. Oxygen in thefunctional group in graphene oxide is negatively charged in a polarsolvent; hence, graphene oxides do not easily aggregate but stronglyinteract with the polar solvent such as NMP. Thus, the functional groupsuch as an epoxy group in the graphene oxide interacts with the polarsolvent, which probably prevents aggregation among graphene oxides,resulting in uniform dispersion of the graphene oxide in a dispersionmedium.

When graphene oxide is used as a raw material of a conductive additiveas described above, the graphene oxide has high dispersibility in adisperse medium but has low conductivity and thus does not function as aconductive additive without any change. For this reason, in forming astorage electrode, after at least an active material and the like andgraphene oxide are mixed, the graphene oxide needs to be reduced to formhighly conductive graphene.

Examples of a method for reducing graphene oxide are reduction treatmentwith heating (hereinafter referred to as thermal reduction treatment),electrochemical reduction treatment performed by application of apotential at which graphene oxide is reduced in an electrolytic solution(hereinafter referred to as electrochemical reduction), and reductiontreatment using a chemical reaction caused with a reducer (hereinafterreferred to as chemical reduction).

In forming a storage battery electrode using graphene oxide as a rawmaterial of a conductive additive, the heat treatment temperature forthermal reduction treatment is limited to lower than or equal to atemperature which exceeds the upper temperature limit of materials ofthe electrode. This is because the graphene oxide cannot be reduceduntil it is mixed with an active material and a binder, in order thathigh dispersibility of the graphene oxide can be kept. On the otherhand, after the graphene oxide is mixed with an active material and abinder, it is not possible to perform heat treatment at a temperaturehigher than or equal to the upper temperature limit of the binder suchas PVDF and higher than or equal to the upper temperature limit of thematerial of a current collector. Thus, it is difficult to sufficientlyreduce graphene oxide in a stage of formation of a storage batteryelectrode.

In the case of performing electrochemical reduction treatment, it isnecessary to sufficiently apply voltage evenly to a storage batteryelectrode under the condition where an electrolytic solution does notdissolve materials of the electrode.

Thus, chemical reduction treatment is employed to reduce graphene oxidein a method for forming a storage battery electrode of the presentinvention. The chemical reduction treatment is performed using areducer.

One embodiment of the present invention is a method for forming astorage battery electrode, which includes the steps of forming a mixtureincluding an active material, graphene oxide, and a binder; providingthe mixture over a current collector; and immersing the mixture providedover the current collector in a polar solvent containing a reducer, sothat the graphene oxide is reduced.

Graphene oxide used as a raw material of a conductive additive can beformed by any of a variety of synthesis methods such as a Hummersmethod, a modified Hummers method, or oxidation of a graphite material.Note that a method for forming a storage battery electrode of thepresent invention is not limited by the degree of separation of grapheneoxides.

Examples of the reducer are ascorbic acid, hydrazine, dimethylhydrazine, hydroquinone, sodium boron hydride (NaBH₄), tetra butylammonium bromide (TBAB), LiAlH₄, ethylene glycol, polyethylene glycol,N,N-diethylhydroxylamine, and a derivative thereof. In particular,ascorbic acid and hydroquinone are preferable to hydrazine and NaBH₄ inthat they are safe due to low reducing ability and utilized industriallywith ease.

A polar solvent can be used as the solvent. Any material can be used forthe polar solvent as long as it can dissolve the reducer. Examples ofthe material of the polar solvent are water, methanol, ethanol, acetone,tetrahydrofuran (THF), dimethylformamide (DMF), N-methylpyrrolidone(NMP), dimethyl sulfoxide (DMSO), and a mixed solution of any two ormore of the above.

Note that heating can facilitate chemical reduction reaction of oneembodiment of the present invention. After drying following the chemicalreduction, heating may further be performed.

A storage battery electrode can be formed by the above formation method.

A method for forming a storage battery electrode including an activematerial layer with high density in which the proportion of conductiveadditive is low and the proportion of the active material is high can beprovided.

The use of the storage battery electrode enables fabrication of astorage battery having high capacity per unit volume of the electrode.

In particular, graphene oxide can be reduced at a reaction temperaturesufficiently lower than that in the case of reducing graphene oxide byheat treatment.

Further, a reducer which has weaker toxicity, e.g., ascorbic acid can beused instead of a highly toxic reducer (i.e., a reducer with highreducing ability) such as hydrazine. The use of such a reducer allowsfabrication of a storage battery electrode through highly safe chemicalreduction of graphene oxide.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A to 1C illustrate a storage battery electrode;

FIG. 2 is a flow chart showing a method for forming an electrode;

FIGS. 3A and 3B illustrate a coin-type secondary battery;

FIG. 4 illustrates a laminated secondary battery;

FIGS. 5A and 5B illustrate a cylindrical secondary battery;

FIG. 6 illustrates electronic devices;

FIGS. 7A to 7C illustrate electronic devices;

FIGS. 8A and 8B illustrate an electronic device;

FIG. 9 shows the discharge characteristics of a battery A;

FIG. 10 shows the discharge characteristics of a comparative battery B;

FIG. 11 shows the discharge characteristics of a comparative battery C;

FIG. 12 is a SEM image of a surface of graphite;

FIG. 13A shows the discharge characteristics of the battery D, and FIG.13B shows the discharge characteristics of the battery A, thecomparative battery B, and a battery D;

FIG. 14 shows the discharge characteristics of the battery A and thecomparative battery B;

FIGS. 15A and 15B show gradient d of the battery A and the comparativebattery B;

FIG. 16A shows the discharge characteristics of the battery A and abattery E, and FIG. 16B shows the discharge characteristics of thecomparative battery B and a battery F;

FIGS. 17A and 17B each show the discharge characteristics of the batteryE and the comparative battery F;

FIG. 18 shows the charge and discharge characteristics of a battery Gand a comparative battery H;

FIG. 19 shows battery characteristics;

FIGS. 20A and 20B each show battery characteristics;

FIGS. 21A and 21B each show battery characteristics;

FIG. 22 shows battery characteristics; and

FIGS. 23A and 23B each are a SEM image of a cross section of a positiveelectrode active material layer.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments and examples will be described with referenceto drawings. However, the embodiments and examples can be implemented inmany different modes, and it will be readily appreciated by thoseskilled in the art that modes and details thereof can be changed invarious ways without departing from the spirit and scope of the presentinvention. Thus, the present invention should not be interpreted asbeing limited to the following descriptions of the embodiments andexamples.

(Embodiment 1)

In this embodiment, a method for forming a storage battery electrode ofone embodiment of the present invention will be described with referenceto FIGS. 1A to 1C and FIG. 2.

FIG. 1A is a perspective view of a storage battery electrode 100, andFIG. 1B is a longitudinal sectional view of the storage batteryelectrode 100. Although the storage battery electrode 100 in the shapeof a rectangular sheet is illustrated in FIG. 1A, the shape of thestorage battery electrode 100 is not limited thereto and may be anyappropriate shape. An active material layer 102 is formed over only onesurface of a current collector 101 in FIGS. 1A and 1B; however, activematerial layers 102 may be formed so that the current collector 101 issandwiched therebetween. The active material layer 102 does notnecessarily need to be formed over the entire surface of the currentcollector 101 and a region that is not coated, such as a region forconnection to an electrode tab, is provided as appropriate.

The current collector 101 can be formed using a highly conductivematerial which is not alloyed with a carrier ion of lithium or the like,such as a metal typified by stainless steel, gold, platinum, zinc, iron,nickel, copper, aluminum, titanium, or tantalum or an alloy thereof.Alternatively, an aluminum alloy to which an element which improves heatresistance, such as silicon, titanium, neodymium, scandium, ormolybdenum, is added can be used. Still alternatively, a metal elementwhich forms silicide by reacting with silicon can be used. Examples ofthe metal element which forms silicide by reacting with silicon includezirconium, titanium, hafnium, vanadium, niobium, tantalum, chromium,molybdenum, tungsten, cobalt, nickel, and the like. The currentcollector 101 can have a foil-like shape, a plate-like shape (sheet-likeshape), a net-like shape, a cylindrical shape, a coil shape, apunching-metal shape, an expanded-metal shape, or the like asappropriate. The current collector 101 preferably has a thickness of 10μm to 30 μm inclusive.

FIG. 1C is a longitudinal sectional view of the active material layer102. The active material layer 102 includes active material particles103, graphenes 104 as a conductive additive, and a binder (notillustrated).

The longitudinal section of the active material layer 102 in FIG. 1Cshows substantially uniform dispersion of the sheet-like graphenes 104in the active material layer 102. The graphenes 104 are schematicallyshown by heavy lines in FIG. 1C but are actually thin films each havinga thickness corresponding to the thickness of a single layer or amultiple layer of carbon molecules. The plurality of graphenes 104 areformed in such a way as to wrap, coat, or be adhered to a plurality ofthe active material particles 103, so that the graphenes 104 makesurface contact with the active material particles 103. Further, thegraphenes 104 are also in surface contact with each other; consequently,the plurality of graphenes 104 form a three-dimensional network forelectric conduction.

This is because graphene oxides with extremely high dispersibility in apolar solvent are used for formation of the graphenes 104. The solventis removed by volatilization from a dispersion medium containing thegraphene oxides uniformly dispersed and the graphene oxides are reducedto give graphenes; hence, the graphenes 104 remaining in the activematerial layer 102 partly overlap with each other and are dispersed suchthat surface contact is made, thereby forming a path for electricconduction.

Unlike a conductive additive in the form of particles, such as acetyleneblack, which makes point contact with an active material, the graphenes104 are capable of surface contact with low contact resistance;accordingly, the electric conduction of the active material particles103 and the graphenes 104 can be improved without an increase in theamount of a conductive additive. Thus, the proportion of the activematerial particles 103 in the active material layer 102 can beincreased. Accordingly, the discharge capacity of a storage battery canbe increased.

FIG. 2 is a flow chart showing a method for forming a storage batteryelectrode of one embodiment of the present invention. First, an activematerial, a binder, and graphene oxide are prepared.

For the active material particles 103, a material into and from whichcarrier ions such as lithium ions can be inserted and extracted is used.The active material particles 103 can be in the form of particles madeof secondary particles with average diameter or diameter distribution,which are obtained in such a way that material compounds are mixed at apredetermined ratio and baked and the resulting baked product iscrushed, granulated, and classified by an appropriate means. Therefore,the active material particles 103 are schematically illustrated asspheres in FIG. 1C; however, the shape of the active material particles103 is not limited to this shape.

The average diameter of a primary particle of the active materialparticles 103 is less than or equal to 500 nm, preferably greater thanor equal to 50 nm and less than or equal to 500 nm. To make surfacecontact with a plurality of the active material particles 103, thegraphenes 104 preferably have sides the length of each of which isgreater than or equal to 50 nm and less than or equal to 100 μm, morepreferably greater than or equal to 800 nm and less than or equal to 20μm.

When a storage battery electrode is formed to be used as a positiveelectrode of a storage battery, a material into and from which lithiumions can be inserted and extracted can be used for the active materialparticles 103; for example, a lithium-containing composite oxide with anolivine crystal structure, a layered rock-salt crystal structure, or aspinel crystal structure can be used. For the active material particles103, a compound such as LiFeO₂, LiCoO₂, LiNiO₂, LiMn₂O₄, V₂O₅, Cr₂O₅, orMnO₂ can be used.

Typical examples of an olivine-type lithium-containing composite oxide(LiMPO₄ (general formula) (M is one or more of Fe(II), Mn(II), Co(II),and Ni(II))) are LiFePO₄, LiNiPO₄, LiCoPO₄, LiMnPO₄, LiFe_(a)Ni_(b)PO₄,LiFe_(a)Co_(b)PO₄, LiFe_(a)Mn_(b)PO₄, LiNi_(a)Co_(b)PO₄,LiNi_(a)Mn_(b)PO₄ (a+b≤1, 0<a<1, and 0<b<1), LiFe_(c)Ni_(d)Co_(e)PO₄,LiFe_(c)Ni_(d)Mn_(e)PO₄, LiNi_(c)Co_(d)Mn_(e)PO₄ (c+d+e≤1, 0<c<1, 0<d<1,and 0<e<1), and LiFe_(f)Ni_(g)Co_(h)Mn_(i)PO₄ (f+g+h+i≤1, 0<f<1, 0<g<1,0<h<1, and 0<i<1).

LiFePO₄ is particularly preferable because it properly satisfiesconditions necessary for the positive electrode active material, such assafety, stability, high capacity density, high potential, and theexistence of lithium ions which can be extracted in initial oxidation(charging).

Examples of a lithium-containing composite oxide with a layeredrock-salt crystal structure are lithium cobalt oxide (LiCoO₂), LiNiO₂,LiMnO₂, Li₂MnO₃, NiCo-containing composite oxide (general formula:LiNi_(x)Co_(1-x)O₂ (0<x<1)) such as LiNi_(0.8)Co_(0.2)O₂,NiMn-containing composite oxide (general formula: LiNi_(x)Mn_(1−x)O₂(0<x <1)) such as LiNi_(0.5)Mn_(0.5)O₂, NiMnCo-containing compositeoxide (also referred to as NMC) (general formula:LiNi_(x)Mn_(y)Co_(1−x−y)O₂ (x>0, y>0, x+y<1)) such asLiNi_(1/3)Mn_(1/3)Co_(1/3)O₂, Li(Ni_(0.8)Co_(0.15)Al_(0.05))O₂, andLi₂MnO₃—LiMO₂ (M=Co, Ni, or Mn).

LiCoO₂ is particularly preferable because of its advantages such as highcapacity and stability in the air higher than that of LiNiO₂ and thermalstability higher than that of LiNiO₂. Examples of a lithium-containingcomposite oxide with a spinel crystal structure are LiMn₂O₄,Li_(1+x)Mn_(2−x)O₄, Li(MnAl)₂O₄, and LiMn_(1.5)Ni_(0.5)O₄.

It is preferable to add a small amount of lithium nickel oxide (LiNiO₂or LiNi_(1−x)MO₂ (M=Co, Al, or the like)) to lithium-containingcomposite oxide with a spinel crystal structure which contains manganesesuch as LiMn₂O₄ because advantages such as minimization of the elutionof manganese and the decomposition of an electrolytic solution can beobtained.

Alternatively, a lithium-containing composite oxide such asLi_((2−j))MSiO₄ (general formula) (M is one or more of Fe(II), Mn(II),Co(II), and Ni(II), 0≤j≤2) can be used for the active material particles103. Typical examples of Li_((2−j))MSiO₄ (general formula) are lithiumcompounds such as Li_((2−j))FeSiO₄, Li_((2−j))NiSiO₄, Li_((2−j))CoSiO₄,Li_((2−j))MnSiO₄, Li_((2−j))Fe_(k)Ni_(l)SiO₄,Li_((2−j))Fe_(k)Co_(l)SiO₄, Li_((2−j))Fe_(k)Mn_(l)SiO₄,Li_((2−j))Ni_(k)Co_(l)SiO₄, Li_((2−j))Ni_(k)Mn_(l)SiO₄ (k+l≤1, 0<k<1,and 0<l<1), Li_((2−j))Fe_(m)Ni_(n)Co_(q)SiO₄,Li_((2−j))Fe_(m)Ni_(n)Mn_(q)SiO₄, Li_((2−j))Ni_(m)Co_(n)Mn_(q)SiO₄(m+n+q≤1, 0<m<1, 0<n<1, and 0<q<1), andLi_((2−j))Fe_(r)Ni_(s)Co_(t)Mn_(u)SiO₄ (r+s+t+u≤1, 0<r<1, 0<s<1, 0<t<1,and 0<u<1).

Still alternatively, a nasicon compound expressed by A_(x)M₂(XO₄)₃(general formula) (A=Li, Na, or Mg, M=Fe, Mn, Ti, V, Nb, or Al, X=S, P,Mo, W, As, or Si) can be used for the active material particles 103.Examples of the nasicon compound are Fe₂(MnO₄)₃, Fe₂(SO₄)₃, andLi₃Fe₂(PO₄)₃. Further alternatively, a compound expressed by Li₂MPO₄F,Li₂MP₂O₇, or Li₅MO₄ (general formula) (M=Fe or Mn), a perovskitefluoride such as NaF₃ or FeF₃, a metal chalcogenide (a sulfide, aselenide, or a telluride) such as TiS₂ or MoS₂, a lithium-containingcomposite oxide with an inverse spinel crystal structure such as LiMVO₄,a vanadium oxide (V₂O₅, V₆O₁₃, LiV₃O₈, or the like), a manganese oxide,an organic sulfur, or the like can be used as the positive electrodeactive material.

In the case where carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions, thefollowing may be used as the positive electrode active material: acompound or a composite oxide which is obtained by substituting analkali metal (e.g., sodium or potassium), an alkaline-earth metal (e.g.,calcium, strontium, or barium), beryllium, or magnesium for lithium inthe lithium compound or the lithium-containing composite oxide.

When a storage battery electrode is formed to be used as a negativeelectrode of a storage battery, a material with which lithium can bedissolved and precipitated or a material into and from which lithiumions can be inserted and extracted can be used for the active materialparticles 103; for example, a lithium metal, a carbon-based material, analloy-based material, or the like can be used.

The lithium metal is preferable because of its low redox potential(3.045 V lower than that of a standard hydrogen electrode) and highspecific capacity per unit weight and per unit volume (3860 mAh/g and2062 mAh/cm³).

Examples of the carbon-based material include graphite, graphitizingcarbon (soft carbon), non-graphitizing carbon (hard carbon), a carbonnanotube, graphene, carbon black, and the like.

Examples of the graphite include artificial graphite such as meso-carbonmicrobeads (MCMB), coke-based artificial graphite, or pitch-basedartificial graphite and natural graphite such as spherical naturalgraphite.

Graphite has a low potential substantially equal to that of a lithiummetal (0.1 V to 0.3 V vs. Li/Li⁺) when lithium ions are inserted intothe graphite (when a lithium-graphite intercalation compound is formed).For this reason, a lithium ion battery can have a high operatingvoltage. In addition, graphite is preferable because of its advantagessuch as relatively high capacity per unit volume, small volumeexpansion, low cost, and safety greater than that of a lithium metal.

For the active material particles 103, an alloy-based material whichenables charge-discharge reaction by alloying and dealloying reactionwith a lithium metal can be used. For example, a material containing atleast one of Al, Si, Ge, Sn, Pb, Sb, Bi, Ag, Au, Zn, Cd, In, Ga, and thelike can be given. Such elements have higher capacity than carbon. Inparticular, silicon has a significantly high theoretical capacity of4200 mAh/g. For this reason, silicon is preferably used as the negativeelectrode active material. Examples of the alloy-based material usingsuch elements include SiO, Mg₂Si, Mg₂Ge, SnO, SnO₂, Mg₂Sn, SnS₂, V₂Sn₃,FeSn₂, CoSn₂, Ni₃Sn₂, Cu₆Sn₅, Ag₃Sn, Ag₃Sb, Ni₂MnSb, CeSb₃, LaSn₃,La₃Co₂Sn₇, CoSb₃, InSb, SbSn, and the like.

Alternatively, for the active material particles 103, an oxide such astitanium dioxide (TiO₂), lithium titanium oxide (Li₄Ti₅O₁₂),lithium-graphite intercalation compound (Li_(x)C₆), niobium pentoxide(Nb₂O₅), tungsten oxide (WO₂), or molybdenum oxide (MoO₂) can be used.

Still alternatively, for the active material particles 103,Li_(3−x)M_(x)N (M=Co, Ni, or Cu) with a Li₃N structure, which is anitride containing lithium and a transition metal, can be used. Forexample, Li_(2.6)Co_(0.4)N₃ is preferable because of high charge anddischarge capacity (900 mAh/g and 1890 mAh/cm³).

A nitride containing lithium and a transition metal is preferably used,in which case lithium ions are contained in the negative electrodeactive material and thus the negative electrode active material can beused in combination with a material for a positive electrode activematerial which does not contain lithium ions, such as V₂O₅ or Cr₃O₈.Note that in the case of using a material containing lithium ions as apositive electrode active material, the nitride containing lithium and atransition metal can be used for the negative electrode active materialby extracting the lithium ions contained in the positive electrodeactive material in advance.

Alternatively, a material which causes a conversion reaction can be usedas the negative electrode active material; for example, a transitionmetal oxide which does not cause an alloy reaction with lithium, such ascobalt oxide (CoO), nickel oxide (NiO), or iron oxide (FeO), may beused. Other examples of the material which causes a conversion reactioninclude oxides such as Fe₂O₃, CuO, Cu₂O, RuOz, and Cr₂O₃, sulfides suchas CoS_(0.89), NiS, or CuS, nitrides such as Zn₃N₂, Cu₃N, and Ge₃N₄,phosphides such as NiP₂, FeP₂, and CoP₃, and fluorides such as FeF₃ andBiF₃. Note that any of the fluorides can be used as a positive electrodeactive material because of its high potential.

As the binder, polyvinylidene fluoride (PVDF) as a typical example,polyimide, polytetrafluoroethylene, polyvinyl chloride,ethylene-propylene-diene polymer, styrene-butadiene rubber,acrylonitrile-butadiene rubber, fluorine rubber, polyvinyl acetate,polymethyl methacrylate, polyethylene, nitrocellulose, or the like canbe used.

The graphene oxide is a raw material of the graphene 104 which serves asa conductive additive later. The graphene oxide can be formed by any ofa variety of synthesis methods such as a Hummers method, a modifiedHummers method, or oxidation of a graphite material. Note that themethod for forming a storage battery electrode of the present inventionis not limited by the degree of separation of the graphene oxides.

For example, in a Hummers method, graphite such as flake graphite isoxidized to give graphite oxide. The obtained graphite oxide is graphitewhich is oxidized in places and thus to which a functional group such asa carbonyl group, a carboxyl group, or a hydroxyl group is bonded. Inthe graphite oxide, the crystallinity of the graphite is lost and thedistance between layers is increased. Therefore, graphene oxide can beeasily obtained by separation of the layers from each other byultrasonic treatment or the like.

The length of one side (also referred to as a flake size) of thegraphene oxide is greater than or equal to 50 nm and less than or equalto 100 μm, preferably greater than or equal to 800 nm and less than orequal to 20 μm. Particularly in the case where the flake size is smallerthan the average diameter of the active material particles 103, surfacecontact with a plurality of the active material particles 103 andconnection between graphenes are difficult, resulting in difficulty inincreasing the electric conductivity of the active material layer 102.

The graphene oxide, the active material, and the binder described aboveare added to a polar solvent such as N-methylpyrrolidone (NMP) ordimethylformamide, and they are mixed to prepare a paste mixture (StepS11). When a material which significantly interacts with graphene oxideis used for the active material particles 103, graphene oxides can bemore evenly dispersed in the active material layer 102.

Note that the amount of graphene oxide is set to 0.1 wt % to 10 wt %inclusive, preferably 0.1 wt % to 5 wt % inclusive, more preferably 0.2wt % to 1 wt % inclusive with respect to the total weight of the mixtureof the graphene oxide, the positive electrode active material, theconductive additive, and the binder. On the other hand, the grapheneobtained after a positive electrode paste is applied to the currentcollector and reduction is performed is included at least at 0.05 wt %to 5 wt % inclusive, preferably 0.05 wt % to 2.5 wt % inclusive, morepreferably 0.1 wt % to 0.5 wt % inclusive with respect to the totalweight of a positive electrode active material layer. This is becausethe weight of the graphene is reduced by almost half due to thereduction of the graphene oxide.

Note that a polar solvent may be further added after the mixing so thatthe viscosity of the mixture can be adjusted. Mixing and addition of apolar solvent may be repeated plural times.

Next, the mixture is formed over one surface of the current collector orformed so that the current collector is sandwiched therebetween by acoating method such as a doctor blade method (Step S12).

The mixture formed over the current collector is dried by a method suchas ventilation drying or reduced pressure (vacuum) drying (Step S13).The drying is preferably performed using a hot wind with a temperatureof 50° C. to 180° C. inclusive. Through this step, the polarity solventcontained in the active material layer 102 is evaporated. Note thatthere is no particular limitation on the atmosphere.

The active material layer 102 is pressed by a compression method such asa roll press method or a flat plate press method so as to beconsolidated (Step S14).

Next, reaction is caused in a solvent containing a reducer (Step S15).Through this step, the graphene oxide included in the active materiallayer is reduced to form the graphene 104. Note that it is possible thatoxygen in the graphene oxide is not necessarily entirely released andpartly remains in the graphene. When the graphene 104 contains oxygen,the proportion of the oxygen is 2% to 20% inclusive, preferably 3% to15% inclusive. This reduction treatment is preferably performed athigher than or equal to room temperature and lower than or equal to 150°C. In the method for forming a storage battery electrode, the reductiontreatment is preferably performed at higher than or equal to roomtemperature and lower than or equal to 150° C.

Examples of the reducer are ascorbic acid, hydrazine, dimethylhydrazine, hydroquinone, sodium boron hydride (NaBH₄), tetra butylammonium bromide (TBAB), LiAlH₄, ethylene glycol, polyethylene glycol,N,N-diethylhydroxylamine, and a derivative thereof.

A polar solvent can be used as the solvent. Any material can be used forthe polar solvent as long as it can dissolve the reducer. Examples ofthe material of the polar solvent are water, methanol, ethanol, acetone,tetrahydrofuran (THF), dimethylformamide (DMF), N-methylpyrrolidone(NMIP), dimethyl sulfoxide (DMSO), and a mixed solution of any two ormore of the above.

After that, washing (Step S16) and drying (Step S17) are performed. Thedrying is preferably performed in a reduced pressure (vacuum) atmosphereor a reduction atmosphere. This drying step is performed at, forexample, 50° C. to 200° C. inclusive in vacuum for 1 hour to 48 hoursinclusive. The drying allows evaporation, volatilization, or removal ofthe polar solvent and moisture in the active material layer 102.

Finally, stamping is carried out so that the current collector and theactive material layer have a predetermined size (Step S18), whereby astorage battery electrode is formed.

Note that heating can facilitate the reduction reaction. After thedrying following the chemical reduction, heating may further beperformed.

In the steps described above, the active material layer 102 is pressedin Step S14; however, pressing may be further performed after thewashing step in Step S16 or after the drying step in Step S17, in whichcase pressing in Step S14 can be skipped.

Through the above steps, the storage battery electrode 100 including theactive material layer 102 where the graphenes 104 are evenly dispersedto the active material particles 103 can be formed.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

(Embodiment 2)

In this embodiment, the structure of a storage battery including astorage battery electrode formed by the formation method described inEmbodiment 1 will be described with reference to FIGS. 3A and 3B, FIG.4, and FIGS. 5A and 5B.

(Coin-Type Storage Battery)

FIG. 3A is an external view of a coin-type (single-layer flat type)storage battery, and FIG. 3B is a cross-sectional view thereof.

In a coin-type storage battery 300, a positive electrode can 301doubling as a positive electrode terminal and a negative electrode can302 doubling as a negative electrode terminal are insulated from eachother and sealed by a gasket 303 made of polypropylene or the like. Apositive electrode 304 includes a positive electrode current collector305 and a positive electrode active material layer 306 provided incontact with the positive electrode current collector 305. A negativeelectrode 307 includes a negative electrode current collector 308 and anegative electrode active material layer 309 provided in contact withthe negative electrode current collector 308. A separator 310 and anelectrolytic solution (not illustrated) are provided between thepositive electrode active material layer 306 and the negative electrodeactive material layer 309.

As the positive electrode 304 and the negative electrode 307, storagebattery electrodes formed by the method for forming a storage battery ofone embodiment of the present invention, which is described inEmbodiment 1, can be used.

As the separator 310, an insulator such as cellulose (paper), orpolyethylene or polypropylene with pores can be used.

As an electrolyte in the electrolytic solution, a material whichcontains carrier ions is used. Typical examples of the electrolyte arelithium salts such as LiPF₆, LiClO₄, LiAsF₆, LiBF₄, LiCF₃SO₃,Li(CF₃SO₂)₂N, and Li(C₂F₅SO₂)₂N. One of these electrolytes may be usedalone or two or more of them may be used in an appropriate combinationand in an appropriate ratio.

Note that when carrier ions are alkali metal ions other than lithiumions, alkaline-earth metal ions, beryllium ions, or magnesium ions,instead of lithium in the above lithium salts, an alkali metal (e.g.,sodium or potassium), an alkaline-earth metal (e.g., calcium, strontium,or barium), beryllium, or magnesium may be used for the electrolyte.

As a solvent of the electrolytic solution, a material in which carrierions can transfer is used. As the solvent of the electrolytic solution,an aprotic organic solvent is preferably used. Typical examples ofaprotic organic solvents include ethylene carbonate (EC), propylenecarbonate, dimethyl carbonate, diethyl carbonate (DEC), γ-butyrolactone,acetonitrile, dimethoxyethane, tetrahydrofuran, and the like, and one ormore of these materials can be used. When a gelled high-molecularmaterial is used as the solvent of the electrolytic solution, safetyagainst liquid leakage and the like is improved. Further, the storagebattery can be thinner and more lightweight. Typical examples of gelledhigh-molecular materials include a silicone gel, an acrylic gel, anacrylonitrile gel, polyethylene oxide, polypropylene oxide, afluorine-based polymer, and the like. Alternatively, the use of one ormore of ionic liquids (room temperature molten salts) which havefeatures of non-flammability and non-volatility as a solvent of theelectrolytic solution can prevent the storage battery from exploding orcatching fire even when the storage battery internally shorts out or theinternal temperature increases owing to overcharging or the like.

Instead of the electrolytic solution, a solid electrolyte including aninorganic material such as a sulfide-based inorganic material or anoxide-based inorganic material, or a solid electrolyte including amacromolecular material such as a polyethylene oxide (PEO)-basedmacromolecular material may alternatively be used. When the solidelectrolyte is used, a separator or a spacer is not necessary. Further,the battery can be entirely solidified; therefore, there is nopossibility of liquid leakage and thus the safety of the battery isdramatically increased.

For the positive electrode can 301 and the negative electrode can 302, ametal having a corrosion-resistant property to a liquid such as anelectrolytic solution in charging and discharging a secondary battery,such as nickel, aluminum, or titanium; an alloy of any of the metals; analloy containing any of the metals and another metal (e.g., stainlesssteel); a stack of any of the metals; a stack including any of themetals and any of the alloys (e.g., a stack of stainless steel andaluminum); or a stack including any of the metals and another metal(e.g., a stack of nickel, iron, and nickel) can be used. The positiveelectrode can 301 and the negative electrode can 302 are electricallyconnected to the positive electrode 304 and the negative electrode 307,respectively.

The negative electrode 307, the positive electrode 304, and theseparator 310 are immersed in the electrolytic solution. Then, asillustrated in FIG. 3B, the positive electrode 304, the separator 310,the negative electrode 307, and the negative electrode can 302 arestacked in this order with the positive electrode can 301 positioned atthe bottom, and the positive electrode can 301 and the negativeelectrode can 302 are subjected to pressure bonding with the gasket 303interposed therebetween. In such a manner, the coin-type storage battery300 can be manufactured.

<Laminated Storage Battery>

Next, an example of a laminated storage battery will be described withreference to FIG. 4.

A laminated storage battery 500 illustrated in FIG. 4 is formed with apositive electrode 503 including a positive electrode current collector501 and a positive electrode active material layer 502, a negativeelectrode 506 including a negative electrode current collector 504 and anegative electrode active material layer 505, a separator 507, anelectrolytic solution 508, and an exterior body 509. The separator 507is provided between the positive electrode 503 and the negativeelectrode 506 in the exterior body 509. The exterior body 509 is filledwith the electrolytic solution 508.

In the laminated storage battery 500 illustrated in FIG. 4, the positiveelectrode current collector 501 and the negative electrode currentcollector 504 also function as terminals for electrical contact with anexternal portion. For this reason, each of the positive electrodecurrent collector 501 and the negative electrode current collector 504is provided so as to be partly exposed on the outside of the exteriorbody 509.

As the exterior body 509 in the laminated storage battery 500, forexample, a laminate film having a three-layer structure where a highlyflexible metal thin film of aluminum, stainless steel, copper, nickel,or the like is provided over a film formed of a material such aspolyethylene, polypropylene, polycarbonate, ionomer, or polyamide, andan insulating synthetic resin film of a polyamide resin, a polyesterresin, or the like is provided as the outer surface of the exterior bodyover the metal thin film can be used. With such a three-layer structure,permeation of an electrolytic solution and a gas can be blocked and aninsulating property and resistance to the electrolytic solution can beobtained.

<Cylindrical Storage Battery>

Next, an example of a cylindrical storage battery will be described withreference to FIGS. 5A and 5B. As illustrated in FIG. 5A, a cylindricalstorage battery 600 includes a positive electrode cap (battery cap) 601on the top surface and a battery can (outer can) 602 on the side surfaceand bottom surface. The positive electrode cap 601 and the battery can602 are insulated from each other by a gasket (insulating gasket) 610.

FIG. 5B is a diagram schematically illustrating a cross section of thecylindrical storage battery. Inside the battery can 602 having a hollowcylindrical shape, a battery element in which a strip-like positiveelectrode 604 and a strip-like negative electrode 606 are wound with astripe-like separator 605 interposed therebetween is provided. Althoughnot illustrated, the battery element is wound around a center pin. Oneend of the battery can 602 is close and the other end thereof is open.For the battery can 602, a metal having a corrosion-resistant propertyto a liquid such as an electrolytic solution in charging and discharginga secondary battery, such as nickel, aluminum, or titanium; an alloy ofany of the metals; an alloy containing any of the metals and anothermetal (e.g., stainless steel); a stack of any of the metals; a stackincluding any of the metals and any of the alloys (e.g., a stack ofstainless steel and aluminum); or a stack including any of the metalsand another metal (e.g., a stack of nickel, iron, and nickel) can beused. Inside the battery can 602, the battery element in which thepositive electrode, the negative electrode, and the separator are woundis interposed between a pair of insulating plates 608 and 609 which faceeach other. Further, a nonaqueous electrolytic solution (notillustrated) is injected inside the battery can 602 provided with thebattery element. As the nonaqueous electrolytic solution, a nonaqueouselectrolytic solution which is similar to those of the above coin-typestorage battery and the laminated power storage device can be used.

Although the positive electrode 604 and the negative electrode 606 canbe formed in a manner similar to that of the positive electrode and thenegative electrode of the coin-type storage battery described above, thedifference lies in that, since the positive electrode and the negativeelectrode of the cylindrical storage battery are wound, active materialsare formed on both sides of the current collectors. A positive electrodeterminal (positive electrode current collecting lead) 603 is connectedto the positive electrode 604, and a negative electrode terminal(negative electrode current collecting lead) 607 is connected to thenegative electrode 606. Both the positive electrode terminal 603 and thenegative electrode terminal 607 can be formed using a metal materialsuch as aluminum. The positive electrode terminal 603 and the negativeelectrode terminal 607 are resistance-welded to a safety valve mechanism612 and the bottom of the battery can 602, respectively. The safetyvalve mechanism 612 is electrically connected to the positive electrodecap 601 through a positive temperature coefficient (PTC) element 611.The safety valve mechanism 612 cuts off electrical connection betweenthe positive electrode cap 601 and the positive electrode 604 when theinternal pressure of the battery exceeds a predetermined thresholdvalue. Further, the PTC element 611, which serves as a thermallysensitive resistor whose resistance increases as temperature rises,limits the amount of current by increasing the resistance, in order toprevent abnormal heat generation. Note that barium titanate(BaTiO₃)-based semiconductor ceramic or the like can be used for the PTCelement.

Note that in this embodiment, the coin-type storage battery, thelaminated storage battery, and the cylindrical storage battery are givenas examples of the storage battery; however, any of storage batterieswith a variety of shapes, such as a sealed storage battery and asquare-type storage battery, can be used. Further, a structure in whicha plurality of positive electrodes, a plurality of negative electrodes,and a plurality of separators are stacked or wound may be employed.

As the positive electrodes and the negative electrodes of the storagebattery 300, the storage battery 500, and the storage battery 600, whichare described in this embodiment, electrodes formed by the method forforming a storage battery electrode of one embodiment of the presentinvention are used. Thus, the discharge capacity of the storagebatteries 300, 500, and 600 can be increased.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

(Embodiment 3)

A storage battery including the storage battery electrode of oneembodiment of the present invention can be used for power supplies of avariety of electrical devices driven by power.

Specific examples of electrical devices each utilizing a storage batteryincluding the storage battery electrode of one embodiment of the presentinvention are as follows: display devices of televisions, monitors, andthe like, lighting devices, desktop personal computers and laptoppersonal computers, word processors, image reproduction devices whichreproduce still images and moving images stored in recording media suchas digital versatile discs (DVDs), portable CD players, portable radios,tape recorders, headphone stereos, stereos, table clocks, wall clocks,cordless phone handsets, transceivers, portable wireless devices, mobilephones, car phones, portable game machines, calculators, portableinformation terminals, electronic notebooks, e-book readers, electronictranslators, audio input devices, video cameras, digital still cameras,toys, electric shavers, high-frequency heating appliances such asmicrowave ovens, electric rice cookers, electric washing machines,electric vacuum cleaners, water heaters, electric fans, hair dryers,air-conditioning systems such as air conditioners, humidifiers, anddehumidifiers, dishwashers, dish dryers, clothes dryers, futon dryers,electric refrigerators, electric freezers, electricrefrigerator-freezers, freezers for preserving DNA, flashlights,electrical tools such as a chain saw, smoke detectors, and medicalequipment such as dialyzers. Further, industrial equipment such as guidelights, traffic lights, belt conveyors, elevators, escalators,industrial robots, power storage systems, and power storage devices forleveling the amount of power supply and smart grid can be given. Inaddition, moving objects driven by electric motors using electric powerfrom the storage batteries are also included in the category ofelectrical devices. Examples of the moving objects are electric vehicles(EV), hybrid electric vehicles (HEV) which include both aninternal-combustion engine and a motor, plug-in hybrid electric vehicles(PHEV), tracked vehicles in which caterpillar tracks are substituted forwheels of these vehicles, motorized bicycles including motor-assistedbicycles, motorcycles, electric wheelchairs, golf carts, boats, ships,submarines, helicopters, aircrafts, rockets, artificial satellites,space probes, planetary probes, and spacecrafts.

In the electrical devices, the storage battery including the storagebattery electrode of one embodiment of the present invention can be usedas a main power supply for supplying enough electric power for almostthe whole power consumption. Alternatively, in the electrical devices,the storage battery including the storage battery electrode of oneembodiment of the present invention can be used as an uninterruptiblepower supply which can supply electric power to the electrical deviceswhen the supply of electric power from the main power supply or acommercial power supply is stopped. Still alternatively, in theelectrical devices, the storage battery including the storage batteryelectrode of one embodiment of the present invention can be used as anauxiliary power supply for supplying electric power to the electricaldevices at the same time as the power supply from the main power supplyor a commercial power supply.

FIG. 6 illustrates specific structures of the electrical devices. InFIG. 6, a display device 700 is an example of an electrical deviceincluding a storage battery 704 including the storage battery electrodeof one embodiment of the present invention. Specifically, the displaydevice 700 corresponds to a display device for TV broadcast receptionand includes a housing 701, a display portion 702, speaker portions 703,and the storage battery 704. The storage battery 704 including thestorage battery electrode of one embodiment of the present invention isprovided in the housing 701. The display device 700 can receive electricpower from a commercial power supply. Alternatively, the display device700 can use electric power stored in the storage battery 704 includingthe storage battery electrode of one embodiment of the presentinvention. Thus, the display device 700 can be operated with the use ofthe storage battery 704 including the storage battery electrode of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

A semiconductor display device such as a liquid crystal display device,a light-emitting device in which a light-emitting element such as anorganic EL element is provided in each pixel, an electrophoresis displaydevice, a digital micromirror device (DMD), a plasma display panel(PDP), or a field emission display (FED) can be used for the displayportion 702.

Note that the display device includes, in its category, all ofinformation display devices for personal computers, advertisementdisplays, and the like besides TV broadcast reception.

In FIG. 6, an installation lighting device 710 is an example of anelectrical device including a storage battery 713 including the storagebattery electrode of one embodiment of the present invention.Specifically, the lighting device 710 includes a housing 711, a lightsource 712, and the storage battery 713. Although FIG. 6 illustrates thecase where the storage battery 713 is provided in a ceiling 714 on whichthe housing 711 and the light source 712 are installed, the storagebattery 713 may be provided in the housing 711. The lighting device 710can receive electric power from a commercial power supply.Alternatively, the lighting device 710 can use electric power stored inthe storage battery 713. Thus, the lighting device 710 can be operatedwith the use of storage battery 713 including the storage batteryelectrode of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that although the installation lighting device 710 provided in theceiling 714 is illustrated in FIG. 6 as an example, the storage batteryincluding the storage battery electrode of one embodiment of the presentinvention can be used in an installation lighting device provided in,for example, a wall 715, a floor 716, a window 717, or the like otherthan the ceiling 714. Alternatively, the storage battery including thestorage battery electrode of one embodiment of the present invention canbe used in a tabletop lighting device or the like.

As the light source 712, an artificial light source which emits lightartificially by using electric power can be used. Specifically, anincandescent lamp, a discharge lamp such as a fluorescent lamp, andlight-emitting elements such as an LED and an organic EL element aregiven as examples of the artificial light source.

In FIG. 6, an air conditioner including an indoor unit 720 and anoutdoor unit 724 is an example of an electrical device including astorage battery 723 including the storage battery electrode of oneembodiment of the present invention. Specifically, the indoor unit 720includes a housing 721, an air outlet 722, and the storage battery 723.Although FIG. 6 illustrates the case where the storage battery 723 isprovided in the indoor unit 720, the storage battery 723 may be providedin the outdoor unit 724. Alternatively, the secondary batteries 723 maybe provided in both the indoor unit 720 and the outdoor unit 724. Theair conditioner can receive electric power from a commercial powersupply. Alternatively, the air conditioner can use electric power storedin the storage battery 723. Particularly in the case where the storagebatteries 723 are provided in both the indoor unit 720 and the outdoorunit 724, the air conditioner can be operated with the use of thestorage battery 723 including the storage battery electrode of oneembodiment of the present invention as an uninterruptible power supplyeven when electric power cannot be supplied from a commercial powersupply due to power failure or the like.

Note that although the split-type air conditioner including the indoorunit and the outdoor unit is illustrated in FIG. 6 as an example, thestorage battery including the storage battery electrode of oneembodiment of the present invention can be used in an air conditioner inwhich the functions of an indoor unit and an outdoor unit are integratedin one housing.

In FIG. 6, an electric refrigerator-freezer 730 is an example of anelectrical device including a storage battery 734 including the storagebattery electrode of one embodiment of the present invention.Specifically, the electric refrigerator-freezer 730 includes a housing731, a door for a refrigerator 732, a door for a freezer 733, and thestorage battery 734. The storage battery 734 is provided in the housing731 in FIG. 6. The electric refrigerator-freezer 730 can receiveelectric power from a commercial power supply. Alternatively, theelectric refrigerator-freezer 730 can use electric power stored in thestorage battery 734. Thus, the electric refrigerator-freezer 730 can beoperated with the use of the storage battery 734 including the storagebattery electrode of one embodiment of the present invention as anuninterruptible power supply even when electric power cannot be suppliedfrom a commercial power supply due to power failure or the like.

Note that among the electrical devices described above, a high-frequencyheating apparatus such as a microwave oven and an electrical device suchas an electric rice cooker require high power in a short time. Thetripping of a breaker of a commercial power supply in use of anelectrical device can be prevented by using the storage batteryincluding the storage battery electrode of one embodiment of the presentinvention as an auxiliary power supply for supplying electric powerwhich cannot be supplied enough by a commercial power supply.

In addition, in a time period when electrical devices are not used,particularly when the proportion of the amount of electric power whichis actually used to the total amount of electric power which can besupplied from a commercial power supply source (such a proportionreferred to as a usage rate of electric power) is low, electric powercan be stored in the storage battery, whereby the usage rate of electricpower can be reduced in a time period when the electrical devices areused. For example, in the case of the electric refrigerator-freezer 730,electric power can be stored in the storage battery 734 in night timewhen the temperature is low and the door for a refrigerator 732 and thedoor for a freezer 733 are not often opened or closed. On the otherhand, in daytime when the temperature is high and the door for arefrigerator 732 and the door for a freezer 733 are frequently openedand closed, the storage battery 734 is used as an auxiliary powersupply; thus, the usage rate of electric power in daytime can bereduced.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

(Embodiment 4)

Next, a portable information terminal which is an example of electricaldevices will be described with reference to FIGS. 7A to 7C.

FIGS. 7A and 7B illustrate a tablet terminal 800 which can be folded.FIG. 7A illustrates the tablet terminal 800 in the state of beingunfolded. The tablet terminal includes a housing 801, a display portion802 a, a display portion 802 b, a display-mode switching button 803, apower button 804, a power-saving-mode switching button 805, and anoperation button 807.

A touch panel area 808 a can be provided in part of the display portion802 a, in which area, data can be input by touching displayed operationkeys 809. Note that half of the display portion 802 a has only a displayfunction and the other half has a touch panel function. However, thestructure of the display portion 802 a is not limited to this, and allthe area of the display portion 802 a may have a touch panel function.For example, a keyboard can be displayed on the whole display portion802 a to be used as a touch panel, and the display portion 802 b can beused as a display screen.

A touch panel area 808 b can be provided in part of the display portion802 b like in the display portion 802 a. When a keyboard displayswitching button 810 displayed on the touch panel is touched with afinger, a stylus, or the like, a keyboard can be displayed on thedisplay portion 802 b.

The touch panel area 808 a and the touch panel area 808 b can becontrolled by touch input at the same time.

The display-mode switching button 803 allows switching between alandscape mode and a portrait mode, color display and black-and-whitedisplay, and the like. The power-saving-mode switching button 805 allowsoptimizing the display luminance in accordance with the amount ofexternal light in use which is detected by an optical sensorincorporated in the tablet terminal. In addition to the optical sensor,other detecting devices such as sensors for determining inclination,such as a gyroscope or an acceleration sensor, may be incorporated inthe tablet terminal.

Although the display area of the display portion 802 a is the same asthat of the display portion 802 b in FIG. 7A, one embodiment of thepresent invention is not particularly limited thereto. The display areaof the display portion 802 a may be different from that of the displayportion 802 b, and further, the display quality of the display portion802 a may be different from that of the display portion 802 b. Forexample, one of the display portions 802 a and 802 b may display higherdefinition images than the other.

FIG. 7B illustrates the tablet terminal 800 in the state of beingclosed. The tablet terminal 800 includes the housing 801, a solar cell811, a charge/discharge control circuit 850, a battery 851, and a DC-DCconverter 852. FIG. 7B illustrates an example where the charge/dischargecontrol circuit 850 includes the battery 851 and the DC-DC converter852. The storage battery including the storage battery electrode of oneembodiment of the present invention, which is described in the aboveembodiment, is used as the battery 851.

Since the tablet terminal can be folded, the housing 801 can be closedwhen the tablet terminal is not in use. Thus, the display portions 802 aand 802 b can be protected, which permits the tablet terminal 800 tohave high durability and improved reliability for long-term use.

The tablet terminal illustrated in FIGS. 7A and 7B can also have afunction of displaying various kinds of data (e.g., a still image, amoving image, and a text image), a function of displaying a calendar, adate, the time, or the like on the display portion, a touch-inputfunction of operating or editing data displayed on the display portionby touch input, a function of controlling processing by various kinds ofsoftware (programs), and the like.

The solar cell 811, which is attached on a surface of the tabletterminal, can supply electric power to a touch panel, a display portion,an image signal processor, and the like. Note that the solar cell 811can be provided on one or both surfaces of the housing 801 and thus thebattery 851 can be charged efficiently.

The structure and operation of the charge/discharge control circuit 850illustrated in FIG. 7B will be described with reference to a blockdiagram of FIG. 7C. FIG. 7C illustrates the solar cell 811, the battery851, the DC-DC converter 852, a converter 853, switches SW1 to SW3, andthe display portion 802. The battery 851, the DC-DC converter 852, theconverter 853, and the switches SW1 to SW3 correspond to the charge anddischarge control circuit 850 in FIG. 7B.

First, an example of operation in the case where electric power isgenerated by the solar cell 811 using external light will be described.The voltage of electric power generated by the solar cell is raised orlowered by the DC-DC converter 852 so that the electric power has avoltage for charging the battery 851. When the display portion 802 isoperated with the electric power from the solar cell 811, the switch SW1is turned on and the voltage of the electric power is raised or loweredby the converter 853 to a voltage needed for operating the displayportion 802. In addition, when display on the display portion 802 is notperformed, the switch SW1 is turned off and the switch SW2 is turned onso that the battery 851 may be charged.

Although the solar cell 811 is described as an example of a powergeneration means, there is no particular limitation on the powergeneration means, and the battery 851 may be charged with any of theother means such as a piezoelectric element or a thermoelectricconversion element (Peltier element). For example, the battery 851 maybe charged with a non-contact power transmission module capable ofperforming charging by transmitting and receiving electric powerwirelessly (without contact), or any of the other charge means used incombination.

It is needless to say that one embodiment of the present invention isnot limited to the electrical device illustrated in FIGS. 7A to 7C aslong as the electrical device is equipped with the storage batteryincluding the storage battery electrode of one embodiment of the presentinvention, which is described in the above embodiment.

(Embodiment 5)

Further, an example of the moving object which is an example of theelectrical devices will be described with reference to FIGS. 8A and 8B.

The storage battery described in the above embodiment can be used as acontrol battery. The control battery can be externally charged byelectric power supply using a plug-in technique or contactless powerfeeding. Note that in the case where the moving object is an electricrailway vehicle, the electric railway vehicle can be charged by electricpower supply from an overhead cable or a conductor rail.

FIGS. 8A and 8B illustrate an example of an electric vehicle. Anelectric vehicle 860 is equipped with a battery 861. The output of theelectric power of the battery 861 is adjusted by a control circuit 862and the electric power is supplied to a driving device 863. The controlcircuit 862 is controlled by a processing unit 864 including a ROM, aRAM, a CPU, or the like which is not illustrated.

The driving device 863 includes a DC motor or an AC motor either aloneor in combination with an internal-combustion engine. The processingunit 864 outputs a control signal to the control circuit 862 based oninput data such as data on operation (e.g., acceleration, deceleration,or stop) of a driver or data during driving (e.g., data on an upgrade ora downgrade, or data on a load on a driving wheel) of the electricvehicle 860. The control circuit 862 adjusts the electric energysupplied from the battery 861 in accordance with the control signal ofthe processing unit 864 to control the output of the driving device 863.In the case where the AC motor is mounted, although not illustrated, aninverter which converts direct current into alternate current is alsoincorporated.

The battery 861 can be charged by external electric power supply using aplug-in technique. For example, the battery 861 is charged through apower plug from a commercial power supply. The battery 861 can becharged by converting the supplied power into DC constant voltage havinga predetermined voltage level through a converter such as an AC-DCconverter. The use of the storage battery including the storage batteryelectrode of one embodiment of the present invention as the battery 861can be conducive to an increase in battery capacity, leading to animprovement in convenience. When the battery 861 itself can be morecompact and more lightweight as a result of improved characteristics ofthe battery 861, the vehicle can be lightweight, leading to an increasein fuel efficiency.

Note that it is needless to say that one embodiment of the presentinvention is not limited to the electrical device described above aslong as the storage battery of one embodiment of the present inventionis included.

This embodiment can be implemented in combination with any of the otherembodiments as appropriate.

EXAMPLE 1

The present invention will be described in detail below with examples.Note that the present invention is not limited to the followingexamples.

(Formation of Graphene Oxide Dispersion)

A dispersion containing graphene oxide (hereinafter referred to as agraphene oxide dispersion) was formed by the following method. First, 4g of graphite (BF-40AK manufactured by Chuetsu Graphite Works Co., Ltd.)and 138 mL of concentrated sulfuric acid were mixed to form a mixedsolution. Then, 18 g of potassium permanganate was added to the mixedsolution while they were stirred in an ice bath. After the ice bath wasremoved and stirring was performed at room temperature for 2 hours, theresulting solution was reacted at 35° C. for 30 minutes, so that a mixedsolution A containing graphite oxide was formed.

FIG. 12 shows a SEM image of a surface of graphite used here.

Next, 276 mL of water was added to the mixed solution A containinggraphite oxide while they were stirred in an ice bath. After theresulting mixed solution was stirred in an oil bath at about 95° C. for15 minutes so that reaction was caused, 400 mL of water and 54 mL ofhydrogen peroxide solution (with a concentration of 30%) were added tothe mixed solution while they were stirred, in order to deactivateunreacted potassium permanganate. Consequently, a mixed solution B wasformed.

Next, suction filtration of the mixed solution B was carried out using amembrane filter with a pore size of 0.45 μm to form a precipitate. Afterthat, a mixed solution which was formed by adding a 3.5% hydrochloricacid to the precipitate and then stirring the mixture was subjected tosuction filtration, so that a precipitate containing the graphite oxidewas formed.

The precipitate containing the graphite oxide was mixed with 4 L ofwater and ultrasonic waves with a frequency of 40 kHz were applied tothe obtained mixed solution for one hour, so that a graphene oxidedispersion was formed. Next, centrifugation was carried out at 9000 rpmto collect precipitated graphene oxide.

A mixed solution C formed by adding 2 L of pure water per about 2.5 g ofthe graphene oxide was centrifugated at 9000 rpm plural times, and thegraphene oxide was washed.

Pure water was added to the graphene oxide again to form a grapheneoxide dispersion.

EXAMPLE 2

(Production of Compound)

A compound obtained by drying the graphene oxide dispersion formed bythe method described in Example 1 is referred to as Comparative Sample1.

L-ascorbic acid was added as a reducer to the compound to prepare agraphene oxide dispersion containing about 0.27 g/L graphene oxide and13.5 g/L ascorbic acid. The obtained mixture left in a dark place atroom temperature for 3 hours was centrifugated to obtain a precipitate,and the precipitate was dried at room temperature in vacuum to produce acompound. This compound is referred to as Sample 1.

The compound was irradiated with light from a high pressure mercury lampfor 3 hours while it was bubbled with N₂ in a water bath. The watertemperature after the light irradiation was 50° C. The reacted grapheneoxide aggregated to form a lump, and the liquid was clear. The lump waswashed with pure water and a precipitate obtained by centrifugation orfiltration was dried at room temperature in vacuum to produce acompound. This compound is referred to as Sample 2.

Similarly, the compound obtained as Comparative Sample 1 to whichL-ascorbic acid was not added was irradiated with light from a highpressure mercury lamp for 3 hours, the mixture was washed with purewater and then centrifugated to obtain a precipitate, and theprecipitate was dried at room temperature in vacuum to produce acompound. This compound is referred to as Comparative Sample 2.

Further, L-ascorbic acid was added as a reducer to the graphene oxidedispersion formed by the method described in Example 1 to prepare agraphene oxide dispersion containing about 0.27 g/L graphene oxide and13.5 g/L ascorbic acid. The graphene oxide dispersion was reacted in ahot-water bath at 80° C. in a dark place for 8 hours. The reactedgraphene oxide aggregated to form a lump, and the liquid was clear. Thelump was washed with pure water, centrifugation or filtration wasperformed, and a precipitate was dried at room temperature in vacuum toproduce a compound. This compound is referred to as Sample 3.

An ethanol solution containing 13.5 g/L ascorbic acid and dispersed 0.27g/L graphene oxide was reacted in a hot-water bath at 60° C. for 4.5hours. Consequently, the reacted graphene oxide aggregated to form alump, and the liquid was clear.

Here, ascorbic acid was used as the reducer of the graphene oxide. Theredox reaction of the ascorbic acid can be expressed by Equation (A-1).

Similarly, the compound obtained as Comparative Sample 1 to whichL-ascorbic acid was not added was reacted in a hot-water bath at 80° C.for 8 hours, washing was performed with pure water, centrifugation wasperformed to obtain a precipitate, and the precipitate was dried at roomtemperature in vacuum to produce a compound. This compound is referredto as Comparative Sample 3.

Further, graphene oxide is synthesized by a Modified Hummers method,dried, and pulverized. Then, baking is performed at 300° C. in vacuumfor 10 hours to obtain a compound. This compound is referred to asComparative Sample 4.

(XPS Analysis)

Evaluation of the compositions and evaluation of the carbon bindingstates based on the chemical shift amount of a ls orbital of a carbonatom of Samples 1 to 3 and Comparative Samples 1 to 4 prepared in theabove manner were carried out by X-ray Photoelectron Spectroscopy (XPS).For XPS, QuanteraSXM manufactured by ULVAC-PHI, INCORPORATED having amonochromatic Al X-ray source (1486.6 eV) was used. Table 1 and Table 2show analysis results.

TABLE 1 Sample C O S Others C/O Comparative Sample 1 65.0 33.7 1.2 0.21.93 Sample 1 69.5 29.8 0.4 0.3 2.33 Sample 2 84.9 15.1 <0.1   — 5.62Comparative Sample 2 68.5 31.2 0.3 — 2.20 Sample 3 86.6 13.2 — 0.2 6.56Comparative Sample 3 68.5 31.0 0.3 0.2 2.21 Comparative Sample 4 87.112.9 — — 6.75 Unit: atomic %

TABLE 2 C—C Sample C═C C—H C—O C═O O═C—O Comparative Sample 1 — 35.948.4 11.6 4.1 Sample 1 — 51.2 36.9 8.6 3.3 Sample 2 65.7 12.9 15.5 3.12.8 Comparative Sample 2 — 47.9 41.2 6.6 4.2 Sample 3 69.8 11.9 12.5 2.83.0 Comparative Sample 3 — 50.3 42.0 4.6 3.0 Comparative Sample 4 47.032.4 11.9 4.3 4.3 Unit: atomic %

Table 1 shows XPS analysis results of the compositions of the samples.The proportions (at. %) of carbon (C), oxygen (O), sulfur (S), and theother elements of the samples are shown. In the rightmost columns, theatomic ratios of carbon atoms to oxygen atoms (C/O) are shown. Sample 1has a greater C/O than Comparative Sample 1 not subjected to reductiontreatment, which suggests the reduction reaction of Sample 1.

On the other hand, as shown in Table 1, Sample 2 and Sample 3 havesignificantly high C/O of 5.62 and 6.56, respectively. Also in theatomic ratio in the Cls binding state shown in Table 2, a highproportion of C(sp2)═C(sp2) bonds are observed. Further, the proportionof sulfur (S) in Sample 2 was less than 0.1 and sulfur in Sample 3 wasnot able to be detected, which implies that the sulfur was released fromthe graphene oxide.

Comparison of Sample 2 and Sample 3 with Comparative Sample 4 formed bythermal reduction shows that the samples have similar compositions inTable 1. This suggests that when the formation method of one embodimentof the present invention is employed, graphene oxide can be reduced evenat a sufficiently low temperature as in the case where heat treatment isperformed.

EXAMPLE 3

In this example, the characteristics of a storage battery formed by themethod for forming a storage battery electrode of one embodiment of thepresent invention are compared with the characteristics of a storagebattery formed without employing the method of the present invention.

(Fabrication of Storage Battery)

A method for forming Electrode A will be described. First, NMP was addedas a solvent to LiFePO₄ to which graphene oxide was added and themixture was kneaded until it had the consistency of thick paste. Afteran NMP solution of PVDF (No. 7300 manufactured by KUREHA CORPORATION)was added as a binder solution to the mixture of graphene oxide andLiFePO₄, NMP was further added as a polar solvent and mixing wasperformed to form slurry. Finally, the ratio of LiFePO₄:grapheneoxide:PVDF in the slurry was 93 wt %:2 wt %:5 wt %. The slurry formed bythe above method was applied to a current collector and dried at 80° C.in the air for 40 minutes, so that Electrode A where an active materiallayer was formed over the current collector was formed. The currentcollector was formed in such a manner that aluminum with a thickness of20 μm was coated with a mixture of graphite and sodium polyacrylate (90wt %:10 wt %) to a thickness of about 1 μm with a doctor blade anddrying was eventually performed at 170° C. in vacuum. The activematerial content was about 7 mg/cm².

The graphene oxide was reduced while Electrode A was dried at 170° C. invacuum for 10 hours. After that, resulting Electrode A was pressed andstamped into a circular shape with a diameter of 12 mm. A 2032-type coinbattery including this electrode as a positive electrode is referred toas Comparative Battery B. In Comparative Battery B, polypropylene (PP)was used as a separator; a lithium metal was used as a negativeelectrode; and an electrolytic solution formed in such a manner thatlithium hexafluorophosphate (LiPF₆) was dissolved at a concentration of1 mol/L in a solution in which ethylene carbonate (EC) and diethylcarbonate (DEC) were mixed at a volume ratio of 1:1 was used. Note thatthe active material content in the positive electrode was about 6 mg/cm²to 7 mg/cm².

Further, Electrode A was immersed in an ethanol solution containing 13.5g/L L-ascorbic acid and they were reacted with each other in a hot-waterbath at 60° C. for 4.5 hours, so that the graphene oxide was reduced.That is to say, the graphene oxide was reduced by chemical reduction.Then, the resulting electrode was immersed in ethanol to be washed.Then, the electrode was dried at 100° C. in vacuum for 10 hours,followed by pressing of it. After that, stamping was performed to form acircular shape with a diameter of 12 mm. The obtained electrode was usedas a positive electrode to fabricate a 2032-type coin battery; thisbattery is referred to as Battery A. Materials of components except thepositive electrode in Battery A were the same as those in ComparativeBattery B. Note that the active material content in the positiveelectrode was about 6 mg/cm² to 7 mg/cm².

Further, for comparison, a battery was fabricated using acetylene black(AB), which is a conventional conductive additive, instead of grapheneobtained by reducing graphene oxide. As acetylene black (AB), a powderyproduct of DENKI KAGAKU KOGYO KABUSHIKI KAISHA was used. The specificsurface area was 68 m²/g and the average particle diameter was 35 nm.The compounding ratio (LiFePO₄:conductive additive (AB):PVDF) in thepositive electrode active material layer was set to 80:15:5. In a mannersimilar to that of the above battery, this positive electrode activematerial layer was formed over an Al current collector to form anelectrode and this electrode is used to fabricate Comparative Battery C.

(Characteristics of Storage Batteries and Comparison of ReductionTreatment of Graphene Oxides)

FIGS. 9 to 11 show measurement results of the constant current dischargecharacteristics of Battery A, Comparative Battery B, and ComparativeBattery C.

FIG. 10 is a graph showing the discharge characteristics of ComparativeBattery B, where the horizontal axis represents discharge capacity(mAh/g) and the vertical axis represents voltage (V). As for ComparativeBattery B, graphene oxide used as a raw material of a conductiveadditive in a positive electrode was thermally reduced by heat treatmentat 170° C. for 10 hours.

FIG. 10 shows three discharge curves of different discharge rates. Acharge and discharge rate C refers to the rate at which a battery ischarged and discharged and is represented by “current (A)÷capacity(Ah)”. For example, the charge and discharge rate in the case ofcharging and discharging a battery having a capacity of 1 Ah with 1 A is1 C, and the charge and discharge rate in the case of charging anddischarging the battery with 10 A is 10 C. The discharge rates for themeasurement were 0.2 C (5 hours are needed to completely discharge thebattery), 1 C, and 10 C. Particularly in the case where the dischargerate was 0.2 C, the plateau potential was about 3.4 V, that was high,and the plateau was maintained until when the discharge capacityexceeded 100 mAh/g. Further, a high discharge capacity of about 150mAh/g was observed.

On the other hand, FIG. 9 shows the discharge characteristics of BatteryA including the storage battery electrode formed by the formation methodof one embodiment of the present invention. The horizontal axisrepresents discharge capacity (mAh/g) and the vertical axis representsvoltage (V). As in the case of Comparative Battery B, measurement wascarried out under the conditions that the discharge rates were 0.2 C (5hours are needed to completely discharge the battery), 1 C, and 10 C.

As shown in FIG. 9, in the case where the discharge rate was 0.2 C, theplateau potential was about 3.4 V, which was high, and was maintaineduntil when the discharge capacity exceeded 100 mAh/g. Further, a highdischarge capacity of about 150 mAh/g was observed. This result is veryclose to that of Comparative Battery B, and the results obtained in thecases where the discharge rates were 1 C and 10 C were also very closeto those of Comparative Battery B.

Thus, although the discharge capacity of Battery A was slightly lessthan that of Comparative Battery B, Battery A, which was obtained usingchemical reduction treatment, was able to have battery characteristicssubstantially the same as those of Comparative Battery B, which wasobtained using thermal reduction treatment. That is to say, theelectrode in Comparative Battery B needed to be subjected to thermaltreatment at no less than 170° C., whereas the electrode in Battery Awhich was of equal quality to that in Comparative Battery B was able tobe formed at lower than 170° C.

In formation of the storage battery electrode used for Battery A,L-ascorbic acid was used as a reducer. L-ascorbic acid has lowerreducing ability and lower toxicity than a reducer such as hydrazine andthus is highly safe for formation of a storage battery electrode. Theabove results show that even when L-ascorbic acid with not high reducingability is used as a reducer, the battery having characteristics similarto those of Comparative Battery B, which is obtained using thermalreduction treatment, was able to be fabricated.

FIG. 11 is a graph showing the discharge characteristics of ComparativeBattery C, where the horizontal axis represents discharge capacity(mAh/g) and the vertical axis represents voltage (V). Measurement wascarried out under the condition that the discharge rate was 0.82 C. Thedischarge curve of Comparative Battery C containing acetylene black as aconductive additive does not have a plateau discharge region and thedischarge capacity of Comparative battery C is small.

The above results show that the storage battery including the storagebattery electrode formed by the method for forming a storage batteryelectrode of one embodiment of the present invention had much morefavorable characteristics than the storage battery including theconventional conductive additive.

In other words, by the method for forming a storage battery electrode ofone embodiment of the present invention, the storage battery electrodeincluding the active material layer with high density in which theproportion of conductive additive is low and the proportion of theactive material was high was able to be formed.

Further, graphene oxide was able to be reduced at a reaction temperaturesufficiently lower than that in the case of reducing graphene oxide byheat treatment.

Further, a reducer which has weaker toxicity, e.g., ascorbic acid wasable to be used instead of a highly toxic reducer such as hydrazine.

EXAMPLE 4

In this example, the characteristics of a storage battery formed by themethod for forming a storage battery electrode of one embodiment of thepresent invention are compared with the characteristics of a storagebattery formed without employing the method of the present invention.Note that in this example, unlike in the case of Battery A described inExample 3, an electrode was immersed in an aqueous solution containingascorbic acid instead of being immersed in an ethanol solutioncontaining ascorbic acid so that graphene oxide was reduced.

(Fabrication of Storage Battery)

Electrode A obtained in Example 3 was immersed in a reducing solutionprepared by dissolving 77 mM ascorbic acid and 73 mM lithium hydroxidein ultrapure water and reaction was caused in a hot-water bath at 60° C.for 30 minutes, so that the graphene oxide was reduced. That is to say,the graphene oxide was reduced by chemical reduction. The resultingelectrode was immersed in ethanol to be washed. Then, the electrode wasdried at 100° C. in vacuum for 10 hours, followed by re-pressing of it.After that, stamping was performed to form a circular shape with adiameter of 12 mm. The obtained electrode was used as a positiveelectrode to fabricate a 2032-type coin battery; this battery isreferred to as Battery D. Materials of components except the positiveelectrode in Battery D were the same as those in Comparative Battery B.

(Comparison of Characteristics and Discharge Rates of Storage Batteries)

FIG. 13A shows measurement results of the constant current dischargecharacteristics of Battery D. The horizontal axis represents dischargecapacity (mAh/g) and the vertical axis represents voltage (V). As in thecase of Comparative Battery B, measurement was carried out under theconditions that the discharge rates were 0.2 C (5 hours are needed tocompletely discharge the battery), 1 C, 5 C, and 10 C.

As shown in FIG. 13A, in the case where the discharge rate was 0.2 C,the plateau potential was about 3.4 V, which was high, and wasmaintained until when the discharge capacity exceeded 100 mAh/g.Further, a high discharge capacity of about 150 mAh/g was observed.

(Comparison 1 of Characteristics and Reduction Treatment of GrapheneOxides of Storage Batteries)

FIG. 13B shows the electric characteristics of Battery A and ComparativeBattery B described in Example 3 and Battery D described in thisexample. The horizontal axis represents discharge capacity (mAh/g) andthe vertical axis represents voltage (V). Measurement was carried outunder the condition that the discharge rate was 1 C. In FIG. 13B, athick solid line shows the electric characteristics of Battery D, thinsolid lines show the electric characteristics of Battery A, and dashedlines show the electric characteristics of Comparative Battery B. Themeasurement of the electric characteristics was carried out using onesample of Battery D, two samples of Battery A, and three samples ofComparative Battery B.

As shown in FIG. 13B, like Battery A, the curve of Battery D has a highplateau potential of about 3.4 V and the plateau was maintained untilwhen the discharge capacity exceeded 100 mAh/g. Further, the dischargecapacity of Battery D was as high as about 150 mAh/g as in the case ofComparative Battery B.

Thus, having discharge capacity equal to or substantially equal to thatof Comparative Battery B, which was obtained using thermal reductiontreatment, and small variations in plateau potential, Battery D, whichwas obtained using chemical reduction treatment, was able to havebattery characteristics more excellent than those of Comparative BatteryB. That is to say, the electrode in Comparative Battery B needed to besubjected to thermal treatment at no less than 170° C., whereas theelectrode in Battery D which was of equal quality to that in ComparativeBattery B was able to be formed at lower than 170° C. Further, it wasfound that it was possible to use an organic solvent and water assolvents of L-ascorbic acid serving as a reducer.

(Comparison 2 of Characteristics and Reduction Treatment of GrapheneOxides of Storage Batteries)

FIG. 14 shows measurement results of the electric characteristics of aplurality of samples of Battery A and Comparative Battery B. Note that acurrent collector in a positive electrode of Battery A was formed insuch a manner that aluminum with a thickness of 20 μm was coated with anabout 1-μm-thick mixture of graphite and sodium polyacrylate (90 wt %:10wt %) by a gravure method and drying was performed at 80° C. in the air.The active material content in the positive electrode was about 7mg/cm².

The horizontal axis represents discharge capacity (mAh/g) and thevertical axis represents voltage (V). Measurement was carried out underthe condition that the discharge rate was 1 C. In FIG. 14, solid linesshow the electric characteristics of Battery A, and dashed lines showthe electric characteristics of Comparative Battery B. The measurementof the electric characteristics was carried out using six samples ofBattery A and six samples of Comparative Battery B.

From FIG. 14, the amount of change (gradient d) in discharge voltagewith respect to the discharge capacity from 60 mAh/g to 20 mAh/g wascalculated. Next, FIG. 15A shows a graph where the gradients d ofComparative Battery B are plotted on the horizontal axis and thegradients d of Battery A are plotted on the vertical axis. FIG. 15B isan enlarged graph of FIG. 15A which shows the gradients d of thebatteries in the range of 0 to 1.2.

In each battery, the resistance of the electrode was lower as thegradient d was smaller. This suggests that a high proportion of grapheneformed by reduction of graphene oxide reduces the resistance of apositive electrode. In FIGS. 15A and 15B, the gradients d of ComparativeBattery B vary because of variations in amount of graphene oxidescontained in positive electrodes among the samples. The variations inelectric characteristics result from a manufacturing process of grapheneoxide. On the other hand, the gradients d are substantially equal to oneanother among the samples of Battery A as compared with the case ofComparative Battery B. This indicates that the samples of Battery Aobtained using chemical reduction treatment had small variations inbattery characteristics, which suggests that chemical reductiontreatment makes it possible to reduce graphene oxides with smallvariations.

EXAMPLE 5

In this example, comparison of the amounts of active materials inpositive electrodes and the battery characteristics of the storagebatteries formed by the method for forming a storage battery electrodeof one embodiment of the present invention will be described.

(Fabrication of Storage Battery)

Battery A was fabricated by a fabrication method similar to that inExample 3. Note that the active material content in the positiveelectrode of Battery A was about 6 mg/cm² to 7 mg/cm².

Another battery was fabricated by the fabrication method of Battery A inExample 3 in which the active material content in a positive electrodewas increased. Here, an electrode used as the positive electrode isreferred to as Electrode B. A method for forming Electrode B will bedescribed. Note that LiPO₄ coated with carbon in a stage of solid phasesynthesis of LiFePO₄ with the use of a raw material to which glucose wasadded was used as LiFePO₄. First, NMP was added as a solvent to LiFePO₄to which graphene oxide was added and the mixture was kneaded until ithad the consistency of thick paste. After an NMP solution of PVDF (No.1100 manufactured by KUREHA CORPORATION) was added as a binder solutionto the mixture of graphene oxide and LiFePO₄, NMP was further added as apolar solvent and mixing was performed to form slurry. Finally, theratio of LiFePO₄:graphene oxide:PVDF in the slurry was 91.4 wt %:0.6 wt%:8 wt %. The slurry formed by the above method was applied to a currentcollector and dried at 80° C. in the air for 40 minutes, so thatElectrode B where an active material layer was formed over the currentcollector was formed. The current collector was formed in such a mannerthat aluminum with a thickness of 20 μm was coated with a mixture ofgraphite and sodium polyacrylate (90 wt %:10 wt %) to a thickness ofabout 1 μm with a doctor blade and drying was eventually performed at80° C. in the air. The active material content in the positive electrodewas about 11 mg/cm².

Next, a positive electrode was formed by performing chemical reductiontreatment on Electrode B as in the case of Battery A described inExample 3. The positive electrode was used to fabricate Battery E. Theactive material content in the positive electrode of Battery E was 8mg/cm².

On the other hand, Comparative Battery B was fabricated by a fabricationmethod similar to that in Example 3. Note that the active materialcontent in a positive electrode of Comparative Battery B was about 6mg/cm² to 7 mg/cm².

Another battery was fabricated by the fabrication method of ComparativeBattery B in Example 3 in which the active material content in apositive electrode was increased. Here, a positive electrode was formedby performing thermal reduction treatment on Electrode B. The positiveelectrode was used to fabricate Comparative Battery F. The activematerial content in the positive electrode of Comparative Battery F was9 mg/cm².

(Comparison of Characteristics and Amounts of Active Materials inPositive Electrodes of Storage Batteries)

FIG. 16A shows measurement results of the constant current dischargecharacteristics of Battery A and Battery E, and FIG. 16B showsmeasurement results of the constant current discharge characteristics ofComparative Battery B and Comparative Battery F. The horizontal axisrepresents discharge capacity (mAh/g) and the vertical axis representsvoltage (V). Measurement was carried out under the condition that thedischarge rates were 5 C and 10 C. In FIGS. 16A and 16B, thick solidlines show the electric characteristics of Battery A and ComparativeBattery B, and dashed lines show the electric characteristics of BatteryE and Comparative Battery F.

FIGS. 16A and 16B show that an increase in the active material contentleads to reduction in discharge capacity of the positive electrode. Whenthe battery characteristics measured under the condition that thedischarge rate was 10 C were compared, the difference between thedischarge capacities of Battery A and Battery E was smaller than thedifference between the discharge capacities of Comparative Battery B andComparative Battery F.

(Comparison of Characteristics of Storage Batteries and ReductionTreatment of Graphene Oxides)

FIGS. 17A and 17B show measurement results of the constant currentdischarge characteristics of Battery E and Comparative Battery F. Thehorizontal axis represents discharge capacity (mAh/g) and the verticalaxis represents voltage (V). Measurement was carried out under thecondition that the discharge rates were 5 C and 10 C. FIG. 17A shows thecharacteristics of the batteries under the condition that the dischargerate was 1 C and FIG. 17B shows the characteristics of the batteriesunder the condition that the discharge rate was 5 C. In FIGS. 17A and17B, a solid line shows the electric characteristics of Battery E, and adashed line shows the electric characteristics of Comparative Battery F.

As shown in FIG. 17A, the curve of Battery E has a high plateaupotential of about 3.4 V and the plateau was maintained until when thedischarge capacity exceeded 100 mAh/g. Further, the discharge capacityof Battery E was as high as about 150 mAh/g, which was significantlyclose to the discharge capacity of Comparative Battery F.

Thus, having discharge capacity equal to or substantially equal to thatof Comparative Battery F and small variations in plateau potential,Battery E, which was obtained using chemical reduction treatment, wasable to have battery characteristics more excellent than those ofComparative Battery F, which was obtained using thermal reductiontreatment. That is to say, the electrode in Comparative Battery F neededthermal treatment at no less than 170° C., whereas the electrode inBattery E which is of equal quality to that in Comparative Battery F wasable to be formed at lower than 170° C.

Next, batteries including negative electrodes different from those ofBattery E and Comparative Battery F were fabricated and theircharacteristics were evaluated.

(Fabrication of Storage Batteries)

Battery G was fabricated using a graphite electrode instead of a lithiumelectrode as a negative electrode by a fabrication method similar tothat of Battery E. Comparative Battery H was fabricated using a graphiteelectrode instead of a lithium electrode as a negative electrode by afabrication method similar to that of Comparative Battery F.

(Comparison of Characteristics and Reduction Treatment of GrapheneOxides of Storage Batteries)

FIG. 18 shows measurement results of the constant current charge anddischarge characteristics of Battery G and Battery H. The horizontalaxis represents discharge capacity (mAh/g) and the vertical axisrepresents voltage (V). Measurement was carried out under the conditionthat the charge and discharge rate was 1 C. Note that solid lines showthe charge and discharge characteristics of Battery G and dashed linesshow the charge and discharge characteristics of Comparative Battery H.

The charge and discharge capacity of Battery G was greater than that ofComparative Battery H. Thus, Battery G, which was obtained usingchemical reduction treatment, was able to have battery characteristicsmore excellent than those of Comparative Battery H, which was obtainedusing thermal reduction treatment. That is to say, the electrode inComparative Battery H needed to be subjected to thermal treatment at noless than 170° C., whereas the electrode in Battery G with excellentcharacteristics was able to be formed at lower than 170° C.

EXAMPLE 6

In this example, evaluation results of power storage device electrodesof embodiments of the present invention will be described.

First, evaluation results of power storage device electrodes containinggraphene oxides will be described.

(Formation of Electrodes)

A method for forming an electrode with the use of the graphene oxidedispersion formed in (Formation of Graphene Oxide Dispersion) describedin Example 1 will be described.

As an active material, a conductive additive, and a binder used for theelectrodes, LiFePO₄, graphene oxide (hereinafter referred to as GO), andPVDF were prepared, respectively. Table 3 shows the compounding ratio ofLiFePO₄:GO:PVDF.

TABLE 3 Compounding Ratio LiFePO₄ [wt %] GO [wt %] PVDF [wt %] ElectrodeC1 94.8 0.2 5 Electrode D1 94.6 0.4 5 Electrode E1 94.4 0.6 5 ElectrodeF1 94.2 0.8 5 Electrode G1 94 1 5 Electrode H1 93 2 5 Electrode I1 92 35 Electrode J1 91 4 5

A method for forming Electrode C1 will be described. First, NMP wasadded as a solvent to LiFePO₄ to which graphene oxide was added and themixture was kneaded until it had the consistency of thick paste. Afteran NMP solution of PVDF (No. 1100 manufactured by KUREHA CORPORATION)was added as a binder solution to the mixture of graphene oxide andLiFePO₄, NMP was further added as a polar solvent and mixing wasperformed to form slurry. The slurry formed by the above method wasapplied to a current collector (20-μm-thick aluminum coated with about1-μm-thick acetylene black) and dried at 80° C. in the air for 40minutes, so that Electrode C1 where an active material layer was formedover the current collector was formed. In addition, Electrodes D1 to J1in which the compounding ratios were different from that in Electrode C1were formed similarly to Electrode C1. The active material content wasabout 9 mg/cm².

(Formation of Electrodes Using Thermal Reduction Treatment)

Electrodes C2 to J2 in which the compounding ratios were equal to thosein Electrodes C1 to J1, respectively, were formed. After that,Electrodes C2 to J2 were subjected to heat treatment at 170° C. in areduced-pressure atmosphere for 10 hours to reduce graphene oxides.

(Evaluation of Discharge Capacities of Batteries)

Then, stamping was performed so that Electrodes C2 to J2 subjected tothermal reduction treatment have circular shapes. Coin Batteries C1 toJ1 were fabricated using the following: Electrodes C2 to J2 subjected tothermal reduction treatment and having circular shapes as respectivepositive electrodes; metallic lithium for negative electrodes; a mixedsolution of ethylene carbonate (EC) and diethyl carbonate (DEC) (with avolume ratio of 1:1) in which lithium hexafluorophosphate (LiPF₆)(concentration: 1 mol/L) was dissolved, as electrolytic solutions; andpolypropylene (PP) for separators.

Next, the discharge capacities of Batteries C1 to J1 were measured. Notethat the discharge rate was 1 C.

FIG. 19 shows discharge capacities per unit amount of the activematerials in Batteries C1 to H1. In FIG. 19, the horizontal axisrepresents discharge capacity (mAh/g) and the vertical axis representsvoltage (V).

FIG. 19 shows that an increase in proportion of graphene oxide containedin the active material layer in the electrode increased the dischargecapacity and the discharge voltage.

The results in FIG. 19 suggest trade-off between the strength of theelectrode and the battery characterictics.

(Formation of Electrodes Using Chemical Reduction Treatment 1)

Electrodes C3 to G3 in which the compounding ratios were equal to thosein Electrodes C1 to G1 with high strength, respectively, were formed.

Then, a reducing solution for reducing graphene oxide contained in theelectrodes was prepared. A reducing solution used for Chemical ReductionTreatment 1 was prepared by dissolving 77 mM ascorbic acid in an ethanolsolution.

Electrodes C3 to G3 were immersed in the obtained reducing solution at60° C. for 4.5 hours to reduce graphene oxides.

FIGS. 23A and 23B are SEM images showing an observed cross section ofElectrode E3. In the SEM images, a plurality of positive electrodeactive material particles are seen. In part of the images, aggregatedpositive electrode active material particles can also be seen. Here,white thread-like or string-like portions correspond to graphenes. Notethat among the graphenes, a multilayer graphene including fewer layersmay fail to be observed in the SEM images. Further, even graphenes whichappear to be apart from each other may be connected through a multilayergraphene including fewer layers which fails to be observed by SEM. Thegraphenes can be seen like a thread or a string in a gap (void) betweenthe plurality of positive electrode active material particles and alsoadheres to the surfaces of the positive electrode active materialparticles. In FIG. 23B, some of the graphenes in the SEM image in FIG.23A are highlighted by heavy lines. The graphenes are found to bethree-dimensionally dispersed in the positive electrode active materialparticles in such a way as to wrap the positive electrode activematerial particles. The graphenes make surface contact with theplurality of positive electrode active material particles while being insurface contact with each other. Thus, in the positive electrode activematerial layer, the graphenes are connected to each other and forms anetwork for electric conduction.

(Evaluation of Discharge Capacities of Batteries)

Then, stamping was performed so that Electrodes C3 to G3 subjected tochemical reduction treatment have circular shapes. Coin Batteries C2 toG2 were fabricated using the following: Electrodes C3 to G3 subjected tochemical reduction treatment and having circular shapes as respectivepositive electrodes; metallic lithium for negative electrodes; a mixedsolution of EC and DEC (with a volume ratio of 1:1) in which LiPF₆(concentration: 1 mol/L) was dissolved, as electrolytic solutions; andPP for separators.

Next, the discharge capacities of Batteries C2 to F2 were measured. Notethat the discharge rate was 1 C.

FIGS. 20A and 20B and FIGS. 21A and 21B show discharge capacities perunit amount of the active materials in Batteries C2 to F2. Forcomparison, discharge capacities per unit amount of the active materialsin Batteries C1 to F1 shown in FIG. 19 are also shown. Specifically,FIG. 20A shows the discharge capacities of Battery C1 and Battery C2,FIG. 20B shows the discharge capacities of Battery D1 and Battery D2,FIG. 21A shows the discharge capacities of Battery E1 and Battery E2,and FIG. 21B shows the discharge capacities of Battery F1 and BatteryF2. In FIGS. 20A and 20B and FIGS. 21A and 21B, the horizontal axisrepresents discharge capacity (mAh/g) and the vertical axis representsvoltage (V).

The results in FIGS. 20A and 20B and FIGS. 21A and 21B show that anincrease in proportion of graphene oxide in Batteries C2 to F2 increasedthe discharge capacity and the discharge voltage as in the results shownin FIG. 19.

Further, FIG. 20B and FIGS. 21A and 21B show that the discharge voltagesof Batteries D2 to F2 including the electrodes subjected to chemicalreduction treatment were much higher than those of Batteries D1 to F1,respectively. FIG. 20A shows that the discharge capacities and thedischarge voltages of Battery C1 and Battery C2 were low.

The results in FIGS. 20A and 20B and FIGS. 21A and 21B suggest thatchemical reduction treatment of the electrode increased the reductionrates of graphene oxides contained in the active material layers of theelectrodes, so that the discharge voltages of the batteries were able tobe increased. Further, the results of Battery C1 and Battery C2 showthat the discharge capacities and the discharge voltages of thebatteries were low in the case where the proportion of graphene oxidewas 0.2 wt %. This is presumably because in the case where theproportion of graphene oxide was 0.2 wt %, a graphene network was noteasily formed even when graphene oxide was reduced.

(Formation of Electrodes Using Chemical Reduction Treatment 2)

Next, Electrode E4 in which the compounding ratio was equal to that inElectrode E1 was formed. Note that a current collector of Electrode E4was formed in such a manner that aluminum with a thickness of 20 μm wascoated with a mixture of acetylene black and PVDF (No. 1100 manufacturedby KUREHA CORPORATION) (50 wt %:50 wt %) to a thickness of 2 μm to 8 μmwith a doctor blade.

Then, a reducing solution for reducing graphene oxide contained in theelectrodes was prepared. A reducing solution used for Chemical ReductionTreatment 2 was prepared by dissolving 77 mM ascorbic acid and 74 mMlithium hydroxide in ultrapure water.

Electrode E4 was immersed in the obtained reducing solution at 60° C.for 30 minutes to reduce graphene oxide.

(Evaluation of Discharge Capacities of Batteries)

Then, Electrode E4 subjected to chemical reduction treatment was stampedinto a circular shape. Coin Battery E3 was fabricated using thefollowing: Electrode E4 subjected to chemical reduction treatment andhaving a circular shape as a positive electrode; metallic lithium for anegative electrode; a mixed solution of EC and DEC (with a volume ratioof 1:1) in which LiPF₆ (concentration: 1 mol/L) was dissolved, as anelectrolytic solution; and PP for a separator.

Next, the discharge capacity of Battery E3 was measured. Note that thedischarge rate was 1 C.

FIG. 22 shows discharge capacity per unit amount of the active materialin Battery E3. For comparison, discharge capacities per unit amount ofthe active materials in Battery E1 in FIG. 19 and in Battery E2 in FIG.21A are also shown. In FIG. 22, the horizontal axis represents dischargecapacity (mAh/g) and the vertical axis represents voltage (V).

The results in FIG. 22 shows that the discharge capacity and thedischarge voltage of Battery E3 were higher than those of Battery E2.

The discharge capacities and the energy densities of the batteriesincluding the electrodes formed using Chemical Reduction Treatment 1 andChemical Reduction Treatment 2 were high.

EXAMPLE 7

In this example, part of a reaction mechanism in which graphene isformed from graphene oxide and a quantification method of the impurityconcentration of lithium iron phosphate (LiFePO₄) will be described.

(Reaction of Ascorbic Acid)

A reaction mechanism in which ascorbic acid reduces graphene oxide sothat graphene is formed can be presumably expressed by Equation (B-1) orEquation (B-2), for example. Note that the reaction of the grapheneoxide at an end portion thereof is shown for simplification; thereaction inside the graphene oxide is similar to that at the end portionthereof because steric hindrance is less likely to occur. Further, acarbonyl group and an epoxy group also exist as functional groups ofgraphene oxide; however, a portion containing many hydroxy groups isshown as an example, here.

Equation (B-1) expresses a reaction mechanism in which ascorbic acidprovides a proton to graphene oxide so that graphene is formed. Grapheneoxide to which a proton is added is dehydrated so that graphene isformed. Note that the reaction velocity depends on a reaction solvent;the reaction velocity in the case of using alcohol as a reaction solventis higher than that in the case of using an aprotic solvent and thereaction velocity in the case of using water as a reaction solvent ishigher than that in the case of using alcohol. Thus, the above reactionmechanism is suggested.

Equation (B-2) expresses a reaction mechanism in which ascorbic acid isadded to graphene oxide to form a composite and then dihydroascorbicacid is released from the formed composite, so that graphene is formed.

(Color of Ascorbic Acid Solution where Synthesized Lithium IronPhosphate is Dispersed)

An iron ion in lithium iron phosphate (LiFePO₄) is divalent and iseasily triply oxidized by oxygen in a synthesis atmosphere to become animpurity.

Trivalent iron ions hinder a battery reaction. Further, the amount oflithium in a positive electrode is reduced; thus, the capacity of abattery is reduced unless lithium is contained in a negative electrode.

As examples of a method for determining an impurity containing trivalentiron ions, an X-ray diffraction method, electron spin resonance, and amagnetic susceptibility measurement method using a superconductingquantum interference device are known.

In this example, a method for easily determining an impurity containingtrivalent iron ions by colorimetric measurement of the color of asolution in which a sample is immersed, that is, a method for sensingdissolved trivalent iron ions will be described. Note that thesensitivity may be increased by further addition of an additive.

Lithium iron phosphate (LiFePO₄) was synthesized in a nitrogenatmosphere; the synthesized LiFePO₄ is referred to as Sample 5. Notethat a signal derived from trivalent iron ions was recognized bymeasurement using electron spin resonance.

The sample 5 was dispersed or immersed in an alcohol solution containingascorbic acid and heated to 60° C. for 4.5 hours. As a result, thealcohol solution where Sample 5 was dispersed was slightly colored inred.

Lithium iron phosphate (LiFePO₄) was synthesized in a nitrogenatmosphere; the synthesized LiFePO₄ is referred to as Sample 6. Notethat a signal probably derived more significantly from trivalent ironions than that in the case of Sample 5 was recognized by measurementusing electron spin resonance.

The sample 6 was dispersed or immersed in an alcohol solution containing77 mM ascorbic acid and heated to 60° C. for 4.5 hours. As a result, thealcohol solution where Sample 6 was dispersed was more deeply colored inred than the alcohol solution where Sample 5 was dispersed.

The crystal structures of the lithium iron phosphate (LiFePO₄) containedin Sample 5 and Sample 6 were analyzed by an X-ray diffraction method.According to the analysis result, there was no difference between Sample5 and Sample 6.

The above results show that the amount of impurities containingtrivalent iron ions which are contained in a composition can beevaluated using the color of an ascorbic acid-containing solution inwhich the composition was dispersed or immersed.

(Quantitative Evaluation Method of Trivalent Iron Ions Contained inPower Storage Device Electrode)

The case of application of graphene oxide contained in a mixture over acurrent collector to a step (Step S15 described in Embodiment 1) wheregraphene oxide is reacted in a solution containing a reducer will bedescribed.

A sample in which a mixture containing a given amount of lithium ironphosphate (LiFePO₄) was formed over a current collector was used as astandard sample and immersed in an ethanol solution containing 77 mMascorbic acid and heated to 60° C. for 4.5 hours. The lithium ironphosphate (LiFePO₄) was synthesized by a solid phase method. The mixturewas prepared so as to contain 2 wt % graphene oxide and 5 wt % PVDF (No.7300 manufactured by KUREHA CORPORATION).

An evaluation sample was formed using lithium iron phosphate (LiFePO₄)synthesized in an atmosphere in which the oxygen concentration is higherthan that in the atmosphere used in synthesis of the lithium ironphosphate (LiFePO₄) used in the standard sample. The evaluation samplewas formed over a current collector and immersed in an ascorbic acidsolution similarly to the standard sample.

When the samples, which are electrodes with the same weight, wereimmersed in reducing solutions with the same volume, there was adifference in color of the reducing solution after the immersion betweenthe evaluation sample and the standard sample. Specifically, thesolution where the evaluation sample was immersed became a red solutionhaving an absorption peak higher than that of the solution where thestandard sample was immersed, at 490 nm. Note that ascorbic acid is aweak reducer; thus, there was not a significant difference in absorbanceafter about one-hour immersion.

The standard sample and the evaluation sample were used to fabricaterespective batteries and the discharge capacities of the batteries weremeasured. The difference in discharge capacity between the batteriesunder the condition that the discharge rate was 0.2 C was about 5 mA/gto 10 mA/g.

Even in the case where the evaluation sample was formed using thermalreduction treatment at 170° C. in a reduced pressure environment for 10hours instead of chemical reduction treatment, there was littledifference in discharge capacity between batteries including thesamples.

The above results suggest that the lithium iron phosphate (LiFePO₄) wasinactivated by the impurity.

This application is based on Japanese Patent Application serial no.2012-136194 filed with the Japan Patent Office on Jun. 15, 2012 andJapanese Patent Application serial no. 2013-054310 filed with the JapanPatent Office on Mar. 15, 2013, the entire contents of which are herebyincorporated by reference.

What is claimed is:
 1. A method for manufacturing a storage battery,comprising the steps of: forming a first mixture comprising an activematerial, graphene oxide and a solvent; evaporating the solvent in thefirst mixture so as to form a second mixture comprising the activematerial and graphene oxide; consolidating the second mixture bypressing after evaporating the solvent; and reducing the graphene oxidein the second mixture to form a graphene, wherein the step of reducingthe graphene oxide is performed in a polar solvent comprising a reducer.2. The method for manufacturing a storage battery, according to claim 1,wherein the graphene comprises oxygen after the step of reducing thegraphene oxide.
 3. The method for manufacturing a storage battery,according to claim 2, wherein a proportion of the oxygen is higher thanor equal to 2 at. % and lower than or equal to 20 at. % in the graphene.4. The method for manufacturing a storage battery, according to claim 1,wherein the reducer comprises at least one of ascorbic acid, hydrazine,dimethyl hydrazine, hydroquinone, tetra butyl ammonium bromide, NaBH₄,LiAlH₄, ethylene glycol, polyethylene glycol, andN,N-diethylhydroxylamine.
 5. The method for manufacturing a storagebattery, according to claim 1, wherein the polar solvent comprises atleast one of water, methanol, ethanol, acetone, tetrahydrofuran,dimethylformamide, N-methylpyrrolidone, and dimethyl sulfoxide.
 6. Themethod for manufacturing a storage battery, according to claim 1,wherein the active material comprises lithium.
 7. The method formanufacturing a storage battery, according to claim 1, wherein the stepof reducing the graphene oxide is performed at higher than or equal toroom temperature and lower than or equal to 150 ° C.
 8. The method formanufacturing a storage battery, according to claim 1, wherein the stepof consolidating the second mixture is performed after the step ofreducing the graphene oxide.
 9. An electronic device comprising thestorage battery according to claim
 1. 10. A method for manufacturing astorage battery, comprising the steps of: forming a first mixturecomprising an active material, graphene oxide, and a solvent;evaporating the solvent in the first mixture so as to form a secondmixture comprising the active material and the graphene oxide;consolidating the second mixture by pressing after evaporating thesolvent; and reducing the graphene oxide in the second mixture to form agraphene, wherein the step of reducing the graphene oxide is performedby immersing the second mixture in a polar solvent comprising a reducer.11. The method for manufacturing a storage battery, according to claim10, wherein the reduced graphene oxide comprises oxygen after the stepof reducing the graphene oxide.
 12. The method for manufacturing astorage battery, according to claim 11 wherein a proportion of theoxygen is higher than or equal to 2 at. % and lower than or equal to 20at. % in the reduced graphene oxide.
 13. The method for manufacturing astorage battery, according to claim 10, wherein the reducer comprises atleast one of ascorbic acid, hydrazine, dimethyl hydrazine, hydroquinone,tetra butyl ammonium bromide, NaBH₄, LiAlH₄, ethylene glycol,polyethylene glycol, and N,N-diethylhydroxylamine.
 14. The method formanufacturing a storage battery, according to claim 10, wherein thepolar solvent comprises at least one of water, methanol, ethanol,acetone, tetrahydrofuran, dimethylformamide, N-methylpyrrolidone, anddimethyl sulfoxide.
 15. The method for manufacturing a storage battery,according to claim 10, wherein the active material comprises lithium.16. The method for manufacturing a storage battery, according to claim10, wherein the step of reducing the graphene oxide is performed athigher than or equal to room temperature and lower than or equal to 150° C.
 17. The method for manufacturing a storage battery, according toclaim 10, wherein the step of consolidating the second mixture isperformed after the step of reducing the graphene oxide.
 18. Anelectronic device comprising the storage battery according to claim 10.19. The method for manufacturing a storage battery, according to claim1, wherein the first mixture further comprises a binder.
 20. The methodfor manufacturing a storage battery, according to claim 10, wherein thefirst mixture further comprises a binder.